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3 S. wine INTRODUCTION In the United States, growing pigs are allowed to con- sume feed ad libitum from weaning to the time that they have reached market weight. The feed intake of gilts and boars to be used for breeding is restricted during the latter part of the growing period. Feed intake of the breeding herd is also restricted, except for that of sows during lactation. Therefore, the definition of voluntary feed intake is restricted in this chapter primarily to the growth period and the period during lactation. Voluntary feed intake is expressed in terms of digest- ible energy (DE) because this is the energy introduced into the biological system and because this minimizes the effects of energy density on feed intake. As a unit expressing feed intake, metabolizable energy (ME) pro- vides little improvement in comparison with DE be- cause with a balanced diet, ME is about 96 percent of DE and is relatively constant (Agricultural Research Council [ARC], 1981~. Many studies have not reported BE concentrations of diets or DE intake, and reported feed intake values have been converted to DE intake by assuming that a corn-soybean meal diet contains 3.2 kcal of DE/g. All references to energy concentration are on an as-fed basis (i.e., 90 percent dry matter). Further- more, only factors affecting the ad libitum consumption of nutritionally adequate diets are considered. Because feed costs are 55 to 65 percent of the cost of producing a market pig, feed intake has been recorded in most nutritional studies. In nutritional studies, how- ever, the data are frequently reported for the entire study period or for periods of several weeks. The data are useful for indicating the effects of a dietary factor with reference to a control factor but do not provide insight into the changes in intake that occur on a day-to- day basis. FEED INTAKE OF GROWING PIGS Suckling Pig The feed intake of the suckling, or nursing, pig is dependent upon the ability of the sow to produce milk. Estimates of milk production have been made by isolat- ing pigs from the sow and allowing frequent nursing periods (1-h intervals). Milk production is measured by weighing the suckling pigs before and after suckling and assuming that the gain in weight is a suitable mea- sure of milk intake. This procedure has been described in detail by Lewis et al. (1978~. The summary of experiments reporting solids intake by nursing pigs are derived from milk yield data (Table 3-1~. These data suggest that milk yield is maximum by 8 to 15 days after the beginning of lactation and that it remains relatively constant thereafter. Most studies in- cluded sows that had completed from two to nine repro- ductive cycles. O' Grady et al. (1973) reported results from the first, second, and third reproductive cycles and observed an increase in milk yield and milk solids during the second and third reproductive cycles. Litter size was larger, however, and daily intake per pig was not mark- edly different among lactation periods. Access to dry feed is frequently provided during the nursing period to supplement the nutrient intake of the nursing pig. During long lactation periods, intake of creep feed may be substantial (Table 3-2~. During short lactation periods, however, intake of creep feed is lim- ited. During the second and third week of lactation, in- take of creep feed is less than 20 g/day/pig, while 70 to 80 g/day/pig may be consumed during the fourth week of lactation (Okai et al., 1976; Shields et al., 1980; Aherne et al., 1982b). Providing creep feed is of little benefit if pigs are weaned at 3 to 4 weeks of age. 25
26 Predicting Feed Intake TABLE 3-l Solids Intake hv Nursing Pies Daily Solids Intake (kg/pig) Day of No. of Standard Lactation experimentsa Mean Maximum Minimum Deviation 4-8 7 8-15 20 16-24 18 24-41 22 0.111 0.132 0.136 0.216 0.136 0.195 0.131 0.183 0.080 0.090 0.095 0.093 0.019 0.035 0.030 0.022 aResults are summarized from the following reports: Aherne and Speer, 1974; Boyd et al., 1982; Coffey et al., 1982; Haught and Speer, 1977; Klaver et al., 1981; Lellis and Speer, 1983; Leonard and Speer, 1983; Lewis and Speer, 1973, 1974, 1975; Mahan et al., 1971; O'Grady et al., 1973; Okai et al., 1976; Rousselow et al., 1979; Rousselow and Speer, 1980; Smith, 1959a,b, 1960,a,b,c, 1961; White et al., 1984. TABLE 3-2 Creep Feed Intake by Nursing Pigs Age (Days) When: Feed Offered Study Reference Whitelaw et al., 1966 Elsley et al., 1969 Lodge, 1969 Lynch, 1977 10 Haught et al., 1977 10 King, 1979 Fowler et al., 1979 Shields et al., 1980 Corley et al., 1983 Weaned Intake (g/day) 258.0 245.0 343.0 8.5 28.4 47.8 82.0 30.0 20.8 21 14 21 10 14 14 56 56 56 31 35 27 42 28 28 Based on the data of Okai et al. (1976), Kuan and Mak Weanlin Pi (1979), and Aherne et al. (1982b), DE intake (DEi) can be g g expressed by the following relationship (Figure 3-1~: DEj= 11.2Day- 151.7, (1) where DEi is expressed in kcal/day and Day is age of the pig (R2 = 0.72~. The standard error of the estimates of the regression coefficient was 1.5 and of the intercept was 35.5. The intake of creep feed is predicted to begin at about 13.5 days of age. Because the data were col- lected only until day 35, the use of the prediction equa- tion should be restricted to a 5-week lactation period. FIGURE 3-1 Digestible energy intake of creep feed. Immediately postweaning (for 2 to 3 weeks), social, physiological, environmental, and dietary stress results in poor feed intake and a reduction in growth rate (Leib- brandt et al., 1975a,c; Okai et al., 1976; Shields et al., 1980; Aherne et al., 1982a; McConnell et al., 1982; La- wrence and Maxwell, 1983; Etheridge et al., 1984~. Postweaning growth check and depression of feed in- take is less severe as pigs are weaned at an older age (Leibbrandt et al., 1975b). In studies in which feed in- take by pigs weaned at 2 to 3 weeks of age has been measured daily, feed intake has been reported to in- crease linearly or at an increasing rate. 300 ~DEj = ~1.2 Days- 151.7 O Okai et al., ~ 976 Aherne et ai., 1982b · Kuan and Mak, 1979 200 c' - LL y ,,, 1 00 o a / ~ T ·' 1 TO $ I I I 10 15 20 25 30 DAY OF LACTATION
Swine 27 McCracken (1980) reported that feed intake of pigs weaned at 10 days of age increased linearly for 28 days at about 23 g/day for diets containing dried skim milk and at about 19 g/day for diets containing cereal grains. McCracken (1981) reported similar linear increases in feed intake of 22 g/day for diets containing dried skim milk and 17 g/day for cereal-based diets. Tullis et al. (1980) reported a linear increase in feed intake of diets of different energy densities of 32 g/day (122 kcal/day) for a high-energy diet and 22 g/day (78 kcal/day) for a low- energy diet by intact male pigs weaned at 21 days of age. Ewan (1983) reported a linear increase in feed intake with corn-soybean meal diets by pigs weaned at 21 days of age. The increase in intake was 32 g/day for a starter diet containing 3.4 kcal/g. While linear increases in feed intake postweaning have been observed (McCracken, 1980, 1981; Tullis et al., 1980; Ewan, 1983), the total daily intake is limited by the physical capacity of the digestive tract, and there- fore the rate of increase must decline as the pig grows. Studies of this portion of the feed intake pattern of the pig have not been reported in detail. Ewan (1983) has summarized experiments that have included 4,597 ob- servations of 1,200 pens of pigs weighing from 5 to 20 kg fed corn-soybean meal diets. The DE content of the diets was assumed to be 3.2 kcal/g. Linear and quadratic regression analysis of the data, removing the effects of pigs or their pens, was conducted with body weight, age, or day of experiment as the dependent variable. The data were best described by the following relation- ship (Figure 3-2~: DEi = 462BW - 9.72 BW~ - i,529, (2) with DEi in kcal/day and body weight (BOO) in kg. The standard error of the estimate of the linear coefficient 5000 4000 3000 By LL a ~ 2000 in 1 000 _~' 0 5 6 was 7.5 and that of the quadratic coefficient was 0.35. The coefficient of determination (R2) was 0.928. The relationship predicts maximum intake at 23.8 kg BW, with a decline thereafter, and suggests that is useful only at less than about 20 kg BW. Growing Pig Headley et al. (1961) described cumulative feed in- take of 1,200 pigs fed corn-soybean meal diets in groups of 5 or 10 per pen. The data were used to develop a curve of diminishing increments between body weight and cumulative feed intake. The relationship obtained was: W = A - (A - Wi) eke, (3) where W is the live weight after consumption of if pounds of feed, A is the maximum weight attainable as a result of growth, Wi is the initial weight at which feed- ing began, e is the base of the natural system of loga- rithms, and k is a constant which determines the decline in efficiency as live weight increases. Cole et al. (1967a) fed diets varying in DE content from 2.9 to 3.9 kcal/g to eight pigs over the weight range of 38 to 105 kg live weight. The intake of DE was ex- pressed as an exponential function of body weight. The observed relationship for pigs in pens was DEi = 575BW° 675. (4) DEi was in kcal/day and was reduced if pigs were in metabolism cages rather than pens. This relationship predicts intake satisfactorily between 30 and 100 kg BW, but it does not provide reasonable estimates if ex- tended beyond the range of the data base. In a series of reports (DeShazer and Teter, 1974; D E j = 462BW - 9.72 BW2 - 1 ,529 ~ . · . . 1 1 1 1 1 1 1 1 1 1 1 1 1 1 7 8 9 10 11 12 13 14 15 16 17 18 19 20 BODY WEIGHT (kg) FIGURE 3-2 Digestible energy intake of pigs between 5 and 20 kg body weight. Mean values (O) and standard deviations (.) of 4,597 observations of 1,200 pens of pigs fed nutritionally ade- quate corn-soybean meal diets are shown (Ewan, 1983). Summaries were made at l-kg increments.
28 Predicting Feed Intake Christianson et al., 1980), a model was developed for predicting swine performance. Maximum energy in- take was defined as a quadratic function of body weight for weights from 10 to 45 kg and as a hyperbolic (arc sinh) function of weight in the range of 45 to 110 kg BW. Maximum energy intake was modified by effects of temperature. The derivations of the intake functions were, however, not documented in these reports. Whittemore (1976) described a model of pig growth in which feed intake was described by an asymptotic rela- tionship with body weight. Other relationships for feed intake were also presented that were linear functions of body weight, metabolic body weight, or days on feed. While Whittemore (1976) did not indicate a preferred function, all of the functions are limited if predictions are to be extended above or below the range in body weight of 20 to 100 kg. ARC (1981) reviewed a number of reports of volun- tary feed intake and used a large set of unpublished data of R.A. Houseman, I. McDonald, R. M. I. Crofts, and V. R. Fowler to derive relationships between body weight and feed intake. The observed relationship was DEi = i, 123BW° 5'. (5) ARC also reported estimates of intake at 4 times mainte- nance by two equations: DEj = 717BW° 63 (6) or FIGURE 3-3 Digestible energy intake as an asymptotic function of BW. Mean values (O) and standard deviations (~) of 8,072 observations of 1,490 pens of pigs fed nutritionally adequate corn-soybean meal diets are shown (Ewan, 1983~. Summaries were made at 5-kg incre- ments. DEi = 13,145(1 - e-0 2o4Bw). (7) ~ 5,000 1 2,000 - 9,000 c' - llJ y of LL No clear function for voluntary feed intake was defined, however, by ARC in that report. In a model to simulate life cycle efficiency of pork production, Tess et al. (1983) estimated feed intake from the sum of the ME required for maintenance and that required for protein and fat deposition. Deposition of protein and fat were estimated from quadratic equa- tions based on age. Ewan (1983) summarized data from 62 experiments involving 8,072 observations of 1,490 pens of pigs fed nutritionally adequate corn-soybean meal diets. Pigs fed individually or pens of pigs were considered as ex- perimental units, and observations of feed intake and body weight with time were summarized. The weight class means of pigs from 4.5 to 117 kg BW were fitted by nonlinear regression (Statistical Analysis System, 1982) to the asymptotic relationship, and the following equation was obtained (Figure 3-31: DEj = 13,162(1 - e-0 "~7'i8W). (a) The standard error of the asymptote was 197 and that of the exponent was 0.0005. The equation is very similar to the relationship reported by ARC (19811. Data were not available to allow classification of the data set by environmental or physiological variables but should re- flect the mean response over a wide variety of environ- mental conditions and physiological states. The asymptotic relationship overestimates the DEi of young pigs (Figure 3-31. The use of the quadratic rela- tionship (Equation 2) described in the previous section on the weanling pig provides a better estimate of DEi by pigs in the weight range from 5 to 10 kg BW. From 10 to · DEE = 13,162 ( 1 - e~° 04 76BW} 6,000 . · ~ .-~:: ·W . · I- - ·iC, ~ 3,000 _ ~ /. O I o 20 40 60 BODY WE I G HT (kg) . . ~ - o _ . 80 100 120
Swine 29 20 kg BW, Equations 2 and 8 predict similar intakes. The asymptotic relationship should be restricted to use for pigs weighing more than 20 kg but could be used for pigs between 10 and 20 kg BW. The basic equation form used to predict voluntary feed intake is dependent on the intake of the mature animal. Most of the data used to develop prediction equations for voluntary feed intake are in the range of data from 20 to 100 kg BW. The asymptotic relationship suggests that feed intake increases to a maximum level and is consistent with the concept of a maximum mature body size. ARC (1981) cites unpublished data of K. L. Blaxter and V. R. Fowler of the Rowett Research Insti- tute, Aberdeen, Scotland. Their data suggests that pigs fed ad libitum to maturity reach maximum intake at about 120 kg and that after 120 kg voluntary intake goes into unstable oscillation with a trend toward a slight decline. Data from experiments summarized by Ewan (1983) for pigs ranging in weight from 10 to 116 kg did not suggest a decline in daily intake at heavier weights and supported an asymptotic relationship. Lee and Bowman (1976) observed increasing feed intake by pigs weighing from 32 to 125 kg. In contrast, Siebrits and Kemm (1982) observed a marked decline in daily feed intake above 70 kg BW by South African Landrace pigs. Daily feed intakes were, however, lower than those observed in other studies. Observations by Mahan and Gerber (1984) for 51 pens of pigs weighing from 56 to 150 kg indicated a slight de- cline in daily intake above 100 kg BW. The report of Giles et al. (1981) also supports a decline in daily intake above 60 kg BW. Friend (1973) reported a decline of 148 g of dry matter per week by gilts allowed ad libitum access to protein and cereal pellets. The body weight of the gilts increased from 125 to 180 kg during the 22- week period. VOLUNTARY FEED INTAKE DURING GESTATION Studies of energy requirements during gestation have indicated that restriction of energy intake results in bet- ter reproductive performance than when ad libitum con- sumption of energy is allowed (National Research Council [NRC], 1979~. NRC (1979) recommended re- striction of intake of bred gilts and sows to 1.8 kg of feed providing 6.12 Mcal of DE (5.76 Mcal of ME) daily. The NRC (1979) recommendations are considerably lower than the voluntary daily intakes of 12.6 to 18 Mcal of DE observed by Friend (1971) for bred gilts and sows. Vol- untary intake of energy declined from mating to farrow- ing at a rate of 40.2 kcal/day for gilts and a rate of 47.8 kcal/day for sows (Friend, 1971~. Sows consumed 13.8 percent more energy postweaning than gilts. VOLUNTARY FEED INTAKE DURING LACTATION During lactation the sow physiologically requires nu- trients to support maintenance and milk production. Milk production has a high priority, and if nutrient in- take is restricted, the sow will draw on body tissue in an attempt to maintain milk production. Provided with vol- untary access to feed containing an adequate supply of nutrients, the sow will consume sufficient quantities of nutrients to maintain body weight and to support milk production. Feed intakes during lactation reported in several studies are summarized in Table 3-3. In studies that reported data for gilts, average feed intake was 4.36 kg providing 14,430 kcal of DE daily. Feed intakes re- ported for sows (two or more parities) averaged 5.17 kg and provided 17,030 kcal of DE daily. Gilts consumed 15 percent less energy during lactation than sows. There is wide variation in the observed daily intakes of DE, from 11,452 to 21,689 kcal/day. This variability reflects a multitude of factors that can influence lactation feed intake. Lactation feed intake is low immediately postfarrow- ing and increases as lactation proceeds. Most investiga- tors have only reported lactation feed intake over the entire lactation period. O 'Grady and Lynch (1978) and Stably et al. (1981b) have reported weekly feed intake, and Stably et al. (1976) reported daily intakes for the first 8 days of lactation and weekly values thereafter. With the data from these reports, the change in feed intake with time after farrowing was estimated to be DEi= 13,400 + 596Day - 17.2Day2, (9) where DEi is in kcal/day and Day is the day of lactation (Figure 3-4~. The values for the standard error of the estimate of the linear coefficient, the quadratic coeffi- cient, and the intercept were 149, 7.0, and 740, respec- tively. The relationship is valid for a lactation period of 28 days or less because the maximum lactation length in the data used was 28 days. The quadratic relationship predicts maximum intake on about day 17 of lactation, with a maximum intake of 18,440 kcal of DE daily. This relationship predicts an average intake of 17,200 heal/ day for 26 days of lactation and compares favorably with the mean response for sows of 17,030 kcal daily re- ported in Table 3-3. FACTORS AFFECTING VOLUNTARY FEED INTAKE Physiological Factors Part of the variability associated with feed intake can be attributed to differences in the genetic base or be
Study Reference Gilts Baker et al., 1969 Elsley et al., 1969 Buitrago et al., 1974 Michel et al., 1980 Mahan and Fetter, 1982 Corley et al., 1983 Average No. of Litters No. of Pigs Weaned 30 Predicting Feed Intake TABLE 3-3 Lactation Digestible Energy Intake Length of Lactation (days) Daily intake kg kcal of DE 224 132 71 161 23 133 6.16 8.20 6.97 8.20 7.10 7.14 21 56 56 28 28 28 4.03 5.61 3.69 4.26 4.80 3.76 12,890 20,859 11,797 13,640 15,360 12,035 Total744 Average 7.30 36.2 4.36 14,430 Maximum 8.20 56 5.61 20,859 Minimum 6.16 21 3.69 11,797 Standard deviation 0.79 15.6 0.73 3,402 Sows Jensen, 196417 8.33 28 4.30 13,771 Frobish et al., 1966141a 8.23 14 3.85 12,328 Holden et al., 1968235e 7.48 14 5.48 18,068 Elsley et al., 1969220 8.80 56 5.83 21,689 Libal and Wahlstrom, 197562 8.20 21 6.46 19,759 Stably et al., 1976468 7.60 21 5.61 17,456 Haught et al., 1977154a 8.10 35 5.79 18,536 Boyd et al., 197874a 6.60 14 4.63 16,434 O'Grady and Lynch, 1978320 8.80 28 5.00 15,767 Michel et al., 198093 8.90 28 5.12 16,378 Pollmann et al., 1981230 8.47 21 5.05 17,135 Pond et al., 198188 7.30 28 3.58 11,452 Seerleyetal., 198196a 8.93 21 5.12 16,857 Stably et al., 1981b204 8.75 21 5.51 18,750 Boyd et al., 1982188a 8.50 21 5.46 19,492 Mahan and Fetter, 198248 8.50 28 6.60 21,120 Yoo and Hann, 1982d50 8.95 28 5.22 17,783 Corley et al., 198379 7.58 28 4.48 13,773 Total2,767 Average 8.22 25.2 5.17 17,030 Maximum 8.95 56 6.60 21,689 Minimum 6.60 14 3.58 11,452 Standard deviation 0.66 9.6 0.80 2,827 aThese studies include data from gilts but do not report results for gilts and sows separately. tween breeds. Table 3-4 compares experiments in gilts consume 4.9 percent less feed than the average. which feed intake for various breeds could be com- Boars also tend to consume less feed than barrows. pared. Within experiments, the pigs of each breed had similar initial and final weights. Duroc and crossbred pigs tended to consume more feed than other breeds. Differences in daily voluntary intake probably occur be tween selected lines within breeds and between differ ent crosses between lines. There are, however, insufficient data to allow quantitation of the effect of these variables on voluntary feed intake. Voluntary feed intake by barrows is greater than the feed intake by gilts. Table 3-5 summarizes comparisons of feed intake between barrows and gilts and indicates that barrows consume 4.9 percent more feed than the average consumption of both barrows and gilts and that From the data reported by Siers (1975), Henry (1969), and Fowler et al. (1981), boars consumed 5.16 percent less feed than barrows. The difference in feed intake by gilts and barrows is dependent on weight and is not observed during the starter, or postweaning, period. Data reported by Cop and Buiting (1977) compared intake of gilts and barrows at 5-kg increments from 25 to 95 kg and can be pressed by the following quadratic relationship: Percent deviation = 0.2142BW - 0.00133BW2 - 4.42, (10) where percent deviation indicates the change from the
Swine 3 1 20,000 1 7,500 - y 1 5,000 At IL 1 2,500 1 0,000 0 0 o · g 0/ ,~0 Or Or Or DEj = 13,400 + 596 Day - 17.2 Day2 ~ Stably et al., 1976 · Stably et al., 1981 b * O'Grady and Lynch, 1978 L I I I- I I 0 5 10 15 20 25 DAY OF LACTATION mean of barrows and gilts (r2 = 0.9941. The values for the standard error of the estimate of the linear coeffi- cient, the quadratic coefficient, and the intercept were 0.0081, 0.00007, and 0.225, respectively. This relation- ship predicts that positive deviations begin at about 25 kg BW, increase to a maximum of 4.2 percent at 80 kg BW, and decline slightly thereafter. Similar responses have been reported by Tjong-A-Hung et al. (1972), Siers (1975), and Christian et al. (1980~. Lee and Bowman (19763 reported a larger difference between barrows and gilts at 48 kg BW than would be predicted by Equa- tion 10 and that the difference declined continuously to 136 kgBW. Friend (1973) reported that the voluntary intake of gilts during estrus decreased to 84.5 percent of the in- take observed between estrus periods, indicating that the physiological state of the animal affects feed con- sumption. Lactating sows fed iodinated casein produce FIGURE 3-4 Digestible energy intake by lactating sows. milk containing higher dry matter, protein, and energy than control sows (Aherne and Speer, 1974~. Feed in- take by sows fed iodinated casein was not affected or tended to decrease (Dudley et al., 1959~. Environmental Factors Temperature is probably the most significant compo- nent of the environment that affects feed intake. NRC (1981) summarized much of the data related to the ef- fects of environment on nutrient requirements. NRC (1981) indicates that there is a zone of thermoneutrality in which heat production is relatively stable. Upper and lower critical temperatures are defined as the upper and lower environmental temperatures where energy must be expended to heat or cool the body. The implication is that between the upper and lower critical temperatures, energy is not required to maintain body temperature TABLE 3-4 Effect of Breed on Daily Digestible Energy Intake Mean DE Intake Study Reference (kcal/day) Duroc HampYorkPCSPC L Cross Johnson et al., 1973 7,104 6.7 0.0-6.3 5.6 Quijandriaetal., 1970 7,264 0.8 -0.82.91.7-3.3 Bruner and Swiger, 1968 7,971 1.6 -3.6-1.2-3.31.6 2.0 Hale and Southwell, 1967 7,270 2.9 -3.1 Bereskin et al., 1976 7,296 3.5 -3.5 Bereskin et al., 1975 7,184 3.8 -3.8 McPheeetal., 1979 6,352 1.8 -1.8 NOTE: Values are the deviations from the mean, in percent. Hamp is Hampshire; York, Yorkshire; PC, Poland China; SPC, Spotted Poland China; L, Landrace; Cross, crossbreed.
32 Predicting Feed Intake TABLE 3-5 Effect of Sex on Daily Digestible Energy Intake Study Reference Clawson et al., 1962 Cole et al., 1967b Hale and Southwell, 1967 Cole et al., 1968 Bruner and Swiger, 1968 Quijandria et al., 1970 Wahlstrom et al., 1971 Wahlstrom et al., 1971 Bereskin et al., 1975 Bereskin et al., 1976 Baird, 1973 Jensen et al., 1973 Fowler et al., 1981 Mahan and Gerber, 1984 Average 7,973 4.91 aPercent deviation is the percent change from the mean intake of barrows and gilts. Deviations are positive for barrows and negative for gilts. For example, in the study of Clawson et al. (1962), barrows in the 17-90-kg weight range ate 2.2 percent more digestible energy than average; the corresponding gilts ate 2.2 percent less. In this case, there is a 4.4 percent difference in digestible energy between the sexes. Mean Weight (kcal of Range(kg) No. DE/day) 17-90 45-91 20-96 59-91 20-94 20-91 18-96 16-96 18-101 20-100 16-100 25-100 30-90 59-135 1,452 1,632 112 128 96 120 144 86 360 72 20 60 7,270 64 10,216 To Deviationa 8,544 2.2 7,500 5.7 5.0 4.6 7,971 5.5 7,264 4.0 7,840 3.3 7,827 5.4 7,168 3.7 7,312 7.7 8,241 4.9 6,736 5.5 8,620 5.5 9,107 5.7 and that feed intake is relatively stable. The zone of thermoneutrality is small for the newborn pig and in- creases with maturity. Heitman et al. (1958) reported that weight gain was maximized at 23 °C for 45-kg pigs and at 16 °C for 159-kg pigs, with a linear response between these two weights. Heitman and Hughes (1949) reported that feed-to-gain ratios were minimized at the same temperatures that gave maximum gain. Verstegen et al. (1978) summa- rized the results of 34 studies on the effects of tempera- ture on feed intake, rate of gain, and feed-to-gain ratio of growing-finishing pigs. They concluded that feed in- take declined at a relatively uniform rate as temperature increased from 5 to 20°C. Between 20 and 25°C the rate of feed intake decline was less than the rate of decline from 5 to 20°C. The temperature range can be extended to 30°C by including data for growing-finishing pigs reported by Nichols et al. (1980), Heitman and Hughes (1949), and Close and Mount (1976, 1978) and data for weanling pigs reported by Le Dividich et al. (1977~. With intake at 15°C established as 100, intake declined linearly from 5 to 30°C by the following relationship: Percent change= 126.3 - 1.65T, (11) where percent change reflects the deviation from feed intake at 15 °C, and T is the reported ambient tempera- ture in °C (Figure 3-5~. The values for the standard error of the estimate of the regression coefficient and the intercept were 0.14 and 2.85, respectively (r2 = 0.826~. Data reported by Nichols et al. (1980) suggest that extreme cold stress drastically increases voluntary feed intake. Intake at 5°C was 119 percent of the intake at 15°C but intake at 0°C was 161 percent of intake at 15°C. Extreme heat stress drastically decreases feed in- take. Expressed as a percentage of intake at 15°C, Heit- man and Hughes (1949) observed intakes of 43.3 percent at 37°C and 21.7 percent at 46°C by pigs weigh- ing between 31 and 65 kg. Nichols et al. (1980) reported that intake at 35°C was 48.1 percent of the intake at 15°C. Temperature stress has a greater effect on heavy (70 to 118 kg) pigs than on light (40 to 70 kg) pigs. Verstegen et al. (1978) reported greater depressions of growth rate for heavy pigs than for light pigs with heat or cold stress. Heitman and Hughes (1949) reported that at 37°C pigs weighing between 75 and 118 kg reduced feed intake about 60 percent more than pigs weighing between 31 and 65 kg. The temperature of the environment is modified by several physical factors, and the term effective ambient temperature (EAT) has been used to describe the tem- perature that the animal experiences (NRC, 1981; Cur- tis' 1983~. This adjusted temperature could be used to correct feed intake for deviations from the optimal tem- perature. The moisture content of the air and the temperature of the environment interact to influence feed intake and performance of pigs. During heat stress the pig depends on evaporative heat loss (panting) to lose heat. Feed intake is reduced and additional energy is required for heat loss and growth depression occurs. For swine the following relationship was suggested for adjusting am- bient air temperature for humidity (Christianson et al., 1980; Curtis, 1983~: (0.65 x dry bulb temperature) + (0.35 x wet bulb temperature). (12) Air movement decreases the EAT by increasing the rates of convective and evaporative heat loss. Mount (1975) concluded that rates of air movement of 0, 0.2, 0.5, and 1.5 m/see change EAT by 0, -4, -7, and -10°C, respectively. In addition, Mount (1975) sug- gested the following changes in EAT for various floor types: straw bedding, 4°C; concrete slats, -5°C; wet
Swine 33 200 150 100 50 _ I · Le Dividich etal., 1977 o o ). ~ to % change = 126.3 - 1.65 T O Verstegen et al., 1978 ~ N ichols et al., 1980 O Close and Mount, 1976 O Close and Mount, 1978 * Heitman and Hughes. 1949 to . 1 1 1 1 1 1 1 6 12 18 TEMPERATURE ( C) surface, -5 to -10°C. Curtis (1983) applied the vari- ables estimated by Mount (1975) and further considered the insulative capacity of the walls of the building to estimate the EAT. Bruce and Clark (1979) evaluated the effects of live weight, feed intake, group size, and ther- mal environment on rate of heat loss and lower critical temperature. The resulting deterministic model pro- vided valid estimates of heat production in the thermal neutral zone and below the critical temperature. From the data of Heitman et al. (1958), the optimal temperature (To) of 23°C for 45-kg pigs and 16°C for 159-kg pigs suggests the following relationship for the effect of body weight on optimal temperature: To = 26 - 0.0614BW. Assuming that the deviation from the To would affect feed intake, as suggested by Verstegen et al. (1978) and others, then: Percent change in DEi = (T., - EAT)0.0165. (14) This correction would appear to be valid between 5 and 30°C, but greater changes in feed intake occur beyond this temperature range. The number of pigs in a pen and the available space per pig both influence feed intake and performance of pigs. Increasing the number of pigs increases EAT by allowing the pigs to huddle in a cool environment and, therefore, reduce heat loss (Mount, 1975~. In a warm environment, reduction of space allowance per pig is detrimental because body heat cannot be easily dissi- pated. Increasing the number of pigs per pen tends to increase the number of aggressive acts, and this effect is further increased if the space allowance per pig is re- duced (Randolph et al., 1981~. 24 30 36 FIGURE 3-5 Effect of temperature on digestible energy intake. With weanling pigs, Lindvall (1981) showed a 9 per cent reduction in feed intake when space allowances were reduced from 0.25 to 0.17 m2/pig and a further reduction of 10 percent if the space allowance was re- duced from 0.17 to 0.13 m2/pig. The North Central Region-89 Committee on Confinement Management of Swine (1984) reported a 7 percent reduction in feed in- take if the space allowance was reduced from 0.23 to 0.14 rn2/pig. Similar observations for growing-finishing pigs indicate that below 1 m2/pig, reduction in the space allowance results in 3 to 10 percent reduction in feed intake, with a tendency for a greater percent decrease as the restriction becomes more severe (Table 3-6~. Kornegay and Notter (1984) reviewed effects of space allowance on the performance of weanling (8 to 21 kg), growing (27 to 54 kg), and finishing (44 to 92 kg) pigs. They derived quadratic relationships between daily feed intake and space allowance within each weight group. By calculating the predicted feed intake and ex- pressing feed intake as a percentage of the maximum predicted intake, the following relationships can be de- rived: Weanling, percent change = 72.27 + 132.40S - 159.54S~ Growing, percent change = 77.25 + 42.93S - 20.25S2 Finishing, percent change = 61.65 + 70.05S - 32.00S2 (15a) (15b) (15c) where percent change is the deviation from optimal feed intake, and S is the space allowance in m2. Kornegay and Notter (1984) reported r2 values of 0.97,0.93, and 0.74 for the original prediction equations for weanling, growing, and finishing pigs, respectively. The space allowances for optimal feed intake were 0.4
34 Predicting Feed Intake TABLE 3-6 Effect of Space Allocation on Daily Feed Intake Space Allowance Weight (m2/pig) FeedIntake(kg) Study/Reference Range(kg) High Low High LowTo Change Gehlbachetal., 1966 14-50 0.36 0.18 1.535 1.370-10.7 Gehlbach et al., 1966 14-50 0.54 0.36 1.600 1.535-4.1 Randolph etal., 1981 16-42 0.66 0.33 1.540 1.480-3.9 NCR-89, 1986 35-57 0.46 0.32 1.830 1.770-3.3 Handlin et al., 1972 35-56 1.06 0.79 2.200 2.040-7.3 Handlin et al., 1972 35-56 1.28 1.06 2.150 2.2002.3 Gehlbachetal., 1966 50-70 0.54 0.36 2.310 2.070-10.4 Gehlbach et al., 1966 50-70 0.72 0.54 2.375 2.310-2.7 Heitman et al., 1961 50-75 1.47 0.84 2.170 2.060-5.1 Heitman et al., 1961 50-77 2.97 1.47 2.320 2.170-6.5 Krideretal., 1975 50-100 0.63 0.43 2.610 2.170-16.8 Gehlbachetal., 1966 70-90 0.72 0.54 2.715 2.520-9.2 NCR-89, 1986 57-96 0.74 0.56 2.680 2.560-4.5 Gehlbachetal., 1966 70-90 0.90 0.72 2.645 2.7152.6 Handlin et al., 1972 57-91 1.06 0.79 3.030 2.850-5.9 Handlin et al., 1972 57-91 1.28 1.06 2.930 3.0303.4 Jensen et al., 1973 Krider et al., 1975 Krider et al., 1975 Jensen et al., 1973 R. cr at l: 25-100 28-98 19-100 25-100 21-89 0.54 0.36 2.145 2.000 -6.7 0.63 0.43 2.410 2.240 -7.0 0.63 0.43 2.165 1.950 -9.9 0.72 0.54 2.200 2.145 -2.5 1.25 0.82 2.200 2.030 -7.7 m2 for weanling, 1.06 m2 for growing, and 1.09 m2 for . . . . finishing pigs. Gehlbach et al. (1966) reported that with constant space allowance (0.36 m2/pig) feed intake was 10 per- cent lower in pens containing 16 pigs as compared with pens containing 8 or 12 pigs during the growing period. During the finishing periods, pens containing 8 pigs con- sumed 6 percent less feed than pigs in pens containing 4 or 6 pigs with a constant space allowance (0.72 m2/pig). Kornegay and Notter (1984) also summarized the ef- fects of varying the number of pigs per pen with a con- stant space allowance. With weanling pigs in a range of 3 to 15 pigs per pen, feed intake decreased 0.92 percent per additional pig in the pen (r2 = 0.97~. With growing pigs in a range of 5 to 32 pigs per pen, feed intake de- creased 0.25 percent per additional pig in the pen (r2 = 0.87~. With finishing pigs in a range of 4 to 30 pigs per pen, feed intake increased 0.32 percent per additional pig in the pen (r2 = 0.921. While it is clear that both group size and space allow- ance influence the consumption of feed by growing pigs, quantitation of the response is not possible with the available data. Individually penned pigs have been re- ported to consume less feed than pigs fed in groups (Cole, 1967a), suggesting that there may be a desirable group size for maximizing feed intake and performance. Nutritional Factors The pig is generally considered to regulate feed in- take to maintain a constant daily intake of energy. Therefore, the weight of feed that is consumed is deter- mined by the energy density of the diet fed. As the energy density of the diet increases, the amount of feed consumed decreases to maintain constant energy in- take. Assuming that the growth rate is maintained, the feed-to-gain ratio improves. Pigs weaned at 14 to 21 days of age and fed diets varying in energy density have a limited ability to regu- late intake based on energy density. Most studies with pigs of this age have involved an increase in the energy density by the addition of fat in an attempt to maintain a rapid growth rate during the postweaning period. Growth rate and feed-to-gain ratios, however, were not improved in many of these studies (Peo et al., 1957; Asplund et al., 1960; Sewell et al., 1961; Eusebio et al., 1965; Sewell and Miller, 1965; Frobish et al., 1969, 1970,1971; Leibbrandtetal.,1975a,c; Menge and Frob- ish, 1976; O' Grady, 1978; Yoo and Han, 1982a; La- wrence and Maxwell, 1983~. If pigs are weaned at greater than 21 days of age and allowed a period to recover from the stress of weaning, increasing energy density has been observed to improve feed efficiency (Sewell and Miller, 1965; Allee et al., 1971; O' Grady, 1978; Aherne et al., 1982a). Among the studies that have reported DE intake in young pigs, Bowland (1964), Allee et al. (1971), and Wilson and Leibholz (1979) did not observe any signifi- cant effects of varying energy density on DE intake.
Swine 35 Leibbrandt et al. (1975a) observed an increase in DE intake as energy density increased. McConnell et al. (1982) observed a quadratic response in DE intake when the energy density was varied from 3,000 to 3,700 kcal/ kg, with maximum DE intake at 3,300 and 3,500 kcal/ kg. Figure 3-6 summarizes the effect of varying the en- ergy density for two weight groups of pigs. Studies with young pigs, with average weights of from 5 to 30 kg, indicate that daily DE intake is relatively stable with diets containing 3.3 to 3.6 kcal/g. With diet energy den- sities of less than 3.3 kcal/g, the young pig is unable to maintain daily DE intake, and daily DE intake de- creases 1,388 kcal/day as energy density decreases 1 kcal/g. With diets containing more than 3.6 kcal/g, the young pig overconsumes energy, and DE intake in- creases at a rate of 183 kcal/day for an increase in en- ergy density of 1 kcal/g. Pigs with average weights greater than 30 kg are also unable to maintain daily DE intake when energy density is less than 3.3 kcal/g. DE intake decreases 2,773 kcal/day for a decrease in energy density of 1 kcal/g. With diets containing 3.3 kcal/g and above, daily DE intake is relatively constant. Deficiencies or excesses of nutrients in diets, in gen- eral, decrease feed intake. These changes require ex- tremes in the dietary concentration of nutrients that are not generally encountered in practical diets. For exam- ple in growing pigs, protein levels of greater than 25 to 30 percent (Wagner et al., 1963; Sugahara et al., 1969) or less than 10 to 12 percent (Robinson et al., 1974) result in significant decreases in DE intake, but levels between these extremes have little effect on DE intake sooo 7000 t=° 6000 Cal y - UJ y 5t,00 4000 3000 2000 fe · ·- · ~ - - c) ~1 1 1 1 1 2.8 3.0 3.2 3.4 3.6 3.8 4.0 . O pigs from 5 to 30 kg · pigs from 30 to 100 kg 1 4.2 ENERGY DENSITY (kcal/g) (Catron et al., 1952; lensen et al., 1955; Gilster and Wahlstrom, 1973; Kornegay et al., 1973~. In contrast, deficiencies of certain amino acids can depress feed in- take rapidly and severely (Robinson, 1975; Montgom- ery et al., 1978~. Antibiotics have been used in swine diets for growth promotion for many years. Hays (1979) summarized the existing research reports of the effects of antibiotics on growth rate and feed efficiency. Stimulation of feed in- take can explain a portion of the response to antibiotics. During the starter phase, pigs fed antibiotics consumed 8 percent more feed than controls. During the growing phase, the response was reduced to 6 percent more than controls, and during the finishing phase, intake was only slightly improved (2 percent) in pigs fed antibiotics. Flavors have been reported to stimulate feed intake by weanling pigs (Campbell, 1976; King, 1979~. Prefer- ence tests have indicated that weanling pigs do have preferences for specific flavors (McLaughlin et al., 1983~. When no choice is given, however, feed intake is not stimulated by a variety of flavors (Kornegay et al., 1979; Ogunbameru et al., 1979; McLaughlin et al., 1983~. Pelleting of feed has resulted in varied responses in feed intake. VanChoubroek et al. (1971) reviewed the effect of pelleting of feed on pig growth rate, feed in- take, and feed efficiency. From the results of 13 trials, they concluded that pelleting resulted in a 9.0 percent reduction in feed intake in weaned pigs. They summa- rized results from 79 trials with finishing pigs and con- cluded that there was a 3.1 percent reduction in feed intake if the feed was pelleted. A portion of the reduc FIGURE 3-6 Effect of energy density on daily DE intake. Data from the following reports were used to derive the figure: Aherne et al., 1982a; Allee et al., 1971; Armstrong and Clawson, 1980; Asplund et al., 1960; Baird et al., 1970, 1975; Bowland, 1964; Campbell, 1977; Clawson, 1967; Corley et al., 1978; Eusebio et al., 1965; Frank et al., 1983; Frobishetal.,1969,1970, 1971; Gilster and Wahlstrom, 1973; Henry, 1969; Hines, 1980a,b; Kass et al., 1980; Keaschall et al., 1983; Kornegay, 1978; Lawrence and Maxwell, 1983; Leibbrandt et al., 1975a,c; Lindemann et al., 1983; McConnell et al., 1982; Menge and Frob- ish, 1976; Mitchell et al., 1965; Moser et al., 1982; Noblet et al., 1980; Noland and Scott, 1960; O'Grady, 1978; Peo et al., 1957; Pond et al., 1962; Rodriguez et al., 1982; Seerley et al., 1978; Sewell and Miller, 1965; Sewell et al., 1961; Sherry et al., 1981; Stahly and Cromwell, 1979; Stahly et al., 1981a; Tribble et al., 1979; Wilson and Liebholz, 1979; Yoo and Han, 1982a,b,c.
36 Predicting Feed Intake tion in feed intake was attributed to a reduction in feed wastage, but this effect was difficult to measure accu- rately. PREDICTING VOLUNTARY FEED INTAKE Daily voluntary feed intake of growing pigs can be predicted by estimating DE consumption under thermo- neutral conditions. The estimated consumption can then be adjusted for physiological, environmental, and nutritional factors. The basic prediction relationships for various weight classes are as follows. For body weights from 5 to 20 kg: DEi = 462BW - 9.72BW~ - 1,529. (2) If weaning stress is considered with weaning at 21 days of age, intake postweaning would increase from 0 at weaning at a rate of 100 kcal/day until intake equals the DEi predicted from the quadratic relationship. For body weights from 10 to 120 kg: DEj = 13,162(1 - e-0 ol7'3Bw). (a) For growing pigs the following adjustment factors can be used. For pigs above 25 kg, adjustments can be made for deviations from equal numbers of barrows and gilts. The deviation in intake can be calculated from the fol- lowing relationship: Percent deviation = 0.2142BW - 0.00133BW: - 4.42. (10) The percent deviation would be subtracted for gilts and added for barrows. Similar deviations can be used for boars and would be subtracted. The adjustment for temperature requires calculation of the EAT, as described by Curtis (1983) or Bruce and Clark (1979), and the To from the following relationship: To = 26 - 0.0614BW. (13) Within the EAT range of 5 to 30°C, adjustment for tem- perature would be as follows: Percent change = 0.0165(T,' - EAT). (14) The adjustment factors for inclusion of antibiotics would be 8 percent from weaning to 16 kg, 6 percent from 16 to 57 kg, and 2 percent from 57 kg to market weight. Adjustment for pelleting would be -9 percent from weaning to 20 kg and -3.1 percent from 20 kg to market. Adjustments for energy density for pigs from weaning to 30 kg are to decrease energy intake by 1,388 kcal/day for each kcal decrease below 3.3 kcal/g, no adjustment between 3.3 and 3.6 kcal/g, and for each kcal above 3.6 kcal/g an increase of 183 kcal/day. For pigs from 30 kg to market, DEi is decreased 2,773 kcal/ day for each kcal/g decrease below 3.3 kcal/g; above 3.3 kcal/g no adjustment is necessary. The adjustment for allowance for various weight groups would be as follows: Weanling (5-20 kg) percent change = 72.27 + 132.40S- 159.54S~ Growing (20-50 kg), percent change = 77.25 + 42.93S- 20.25S2 Finishing (50-100 kg), percent change = 61.65 + 70.0S- 32.00S~ (15a) (15b) (15c) The adjustment for the number of pigs per pen would be - 0.92 percent per pig for weanling pigs, - 0.25 percent per pig for growing pigs, and 0.32 percent per pig for r. . . nnls n1ng plgS. For lactating sows during a lactation period of 28 days or less, DEi can be estimated from the following rela- tionship: DEj= 13,400 + 596Day - 17.2Day~ (9) The predicted DE intake can be adjusted for tempera- ture as described above for growing pigs. EXAMPLE PREDICTIONS A pelleted diet containing 3.3 kcal of DE/g and antibi- otics is fed to a 5-kg pig in an environment with an EAT of 25°C. Predicted intake: DEi= 462~5) - 9.72 (52) _ 1,529 = 538 kcal/day Temperature adjustment: To = 26 - 0.0614~5) = 25.7 (538~[0.0165~25.7 - 25~] = 6 kcal/day Antibiotic adjustment: (5381~0.08) = 43 kcal/day Pelleting adjustment: (538~-0.09) = -48 kcal/day Net intake: 538 + 6 + 43 - 48 = 539 kcal/day A diet containing 3.0 kcal of DE/g is fed to a 50-kg barrow in an environment with an EAT of 15°C. Predicted intake: DEi = 13,162~1 - et-00~76~50~) = 7,702 kcal/day Sex adjustment: Percent deviation = 0.2142~50) - 0.00133~50~2 4.42 = 2.965 [7,702~0.02965) = 228 7,702 + 228 = 7,930 Temperature adjustment: To = 26 - 0.0614~50) = 22.9 (7,930~[0.0165~22.9 - 15~] = 1,034 kcal/day
Swine 37 Energy density adjustment: (3.3 - 3.0~-2,773) = -832 Net intake: 7,702 + 228 + 1,034 - 832 = 8,132 kcal/day A diet containing 3.3 kcal of DE/g is fed to a pen contain- ing 6 gilts and 14 barrows weighing 70 kg and in an environment with an EAT of 15°C. Predicted intake: DEi = 13,162~1 - e<-00~76~70~) = 9,323 kcal/day Adjustment for sex: Percent deviation = 0.2142~70) - 0.00133~702) 4.42 = 4.057 9,323~0.04057) = 378 kcal/day DEi of barrows = 9,323 + 378 = 9,701 DEi of gilts = 9,323 - 378 = 8,945 Pen DEi: 0.7~9,701) + 0.3~8,945) = 9,474 Adjustment for number of pigs for pen: 20~0.0032) = 0.064 (9,474~0.064) = 606 Adjustment for temperature: To = 26 - 0.0614~70) = 21.7 9,474[0.0165 (21.7 - 15~] = 1,047 Net average intake: 9,474 + 606 + 1,047 = 11,127 kcal/day A 200-kg sow on day 7 of lactation is fed a diet contain- ing 3.3 kcal/g in an environment with an EAT of 25°C. Predicted intake: DEi= 13,400 + 596~7) - 17.2~72) = 16,729 Temperature adjustment: To = 26 - 0.0614~200) = 13.7 16,729 [0.0165~13.7 - 25~] = -3,119 Net intake: 16,729 - 3,119 = 13,610 Validation While attempts have been made in this chapter to quantitate a number of variables that affect the volun- tary intake of pigs, it should be recognized that addi- tional factors influence voluntary feed intake. In attempts to validate the relationships proposed in this chapter, data that were obtained from experiments not used to develop the relationships were used. Predicted intakes were calculated from initial body weight and growth rate on a daily basis by pen and adjusted for factors for which information was available. Data re- ported by Nienaber (1981) were available and involved variables of energy density and temperature for pigs weighing from 5 to 30 kg. Intake was overpredicted by 16 percent and may have been because the pigs were maintained in calorimeters during a portion of the stud ies. Actual measured energy values were used for the feeds and may have been lower than the actual values. There was no indication of systematic errors in the pre- dicted values. Data from studies reported by Kinyamu (1985) for pigs (5-20 kg BW) fed diets varying in energy density were also compared to predicted intake. Pre- dicted intakes were 91 and 96 percent of actual values, and no systematic pattern was apparent in the differ- ences between actual and predicted values. Data re- ported by Mahan and Gerber (1984) were also compared to predicted values. The pigs in this study were fed from 60 to 136 kg (beyond the range of the data used to de- velop the basic equation), and the predicted values were 22 percent greater than actual intakes for gilts and 15 percent greater for barrows. Mahan and Gerber (1984) observed a decline in daily intake at heavier weights, and this decline did not occur with the proposed rela- tionship. REFERENCES Agricultural Research Council. 1981. The Nutrient Requirements of Pigs. Slough, England: Commonwealth Agricultural Bureaux. Aherne, F. X., and V. C. Speer.1974. Energy and nitrogen metabolism and performance responses of lactating sows fed thyroprotein. J. Anim. Sci. 38:310. Aherne, F. X., V. Danielsen, and H. E. Nielsen. 1982a. Fat utilization by three-week weaned pigs. Acta Agric. Scand. 32:151. Aherne, F. X., V. Danielsen, and H. E. Nielsen. 1982b. The effects of creep feeding on pre- and post-weaning pig performance. Acta Agric. Scand. 32:155. Allee, G. L., D. H. Baker, and G. A. Leveille. 1971. Fat utilization and lipogenesis in the young pig. J. Nutr. 101:1415. Armstrong, W. D., and A. J. Clawson. 1980. Nutrition and manage- ment of early weaned pigs: Effect of increased nutrient concentra- tions and (or) supplemental liquid feeding. J. Anim. Sci. 50:377. Asplund, J. M., R. H. Grummer, and P. H. Phillips. 1960. Stabilized white grease and corn oil in the diet of baby pigs. J. Anim. Sci. 19:709. Baird, D. M.1973. Influence of pelleting swine diets on metabolizable energy, growth and carcass characteristics. J. Anim. Sci. 36:516. Baird, D. M., H. C. McCampbell, and J. R. Allison. 1970. Levels of crude fiber with constant energy levels for growing-finishing swine using computerized rations. J. Anim. Sci. 31:518. Baird, D. M., H. C. McCampbell, and J. R. Allison. 1975. Effect of levels of crude fiber, protein and bulk in diets for finishing hogs. J. Anim. Sci. 41:1039. Baker, D. H., D. E. Becker, H. W. Norton, C. E. Sasse, A. H. Jensen, and B. G. Harmon. 1969. Reproductive performance and progeny development in swine as influenced by feed intake during preg- nancy. J. Nutr. 97:489. Bereskin, B., R. J. Davey, W. H. Peters, and H. O. Hetzer. 1975. Genetic and environmental effects and interactions in swine growth and feed utilization. J. Anim. Sci. 40:53. Bereskin, B., R. J. Davey, and W. H. Peters. 1976. Genetic, sex and diet effects on pig growth and feed use. J. Anim. Sci. 43:977.
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