4
Reproduction

GESTATION

Meeting the nutrient requirements of pregnant female cattle is important to ensuring an adequate nutrient supply for proper growth and development of the fetus and to ensuring that the female is in adequate body condition to calve and lactate, to rebreed within 80 days after calving, and to provide, in the case of the 2- or 3-year-old heifer, adequate nutrients for continued growth. This section will concentrate on nutrient requirements for pregnancy—in particular, energy and protein—in cattle and some of the factors affecting those requirements.

For lack of information to the contrary, it is generally assumed that nutrient needs for pregnancy are proportional to birth weight of the calf. Thus, it is assumed that factors that affect calf birth weight have a proportional affect on nutrient requirements during pregnancy. Factors known to affect calf birth weight include breed of sire, breed of dam, heterosis, parity of the dam, number of fetuses, sex of the fetus, environmental temperature, and nutrition of the dam (Ferrell, 1991a).

Of the factors affecting calf birth weight, breed or genotype of the sire, dam, or calf generally has the greatest influence (Andersen and Plum, 1965). Typical birth weight of calves of various breeds are listed in Table 4–1. Birth weights of calves in one study differed by as much as 18 kg (Agricultural and Food Research Council, 1990). Ranges to 10 kg were reported for mean birth weights of calves of different breeds typically used for beef production in the United States (Beef Improvement Federation, 1990) or those of crossbred calves from Angus and Hereford dams (Gregory et al., 1982; Cundiff et al., 1988). Heterosis, resulting in increased birth weight, is generally about 6 to 7 percent when Bos taurus breeds are crossed, less (0 or negative) when Bos taurus sires are crossed on Bos indicus dams, but considerably higher (20 to 25 percent) when the reciprocal mating is made (Ellis et al., 1965; Long, 1980;

TABLE 4–1 Estimated Birth Weight of Calves of Different Breeds or Breed Crosses, kg

Breed

BIF

AFRC

MARC

Angus

31

26

35

Brahman

31

41

Braford

36

Brangus

33

Braunvieh

39

Charolais

39

43

40

Chianina

41

Devon

32

34

Galloway

36

Gelbvieh

39

39

Hereford

36

35

37

Holstein

43

Jersey

25

31

Limousin

37

38

39

Longhorn

33

Maine-Anjou

40

41

Nellore

40

Piedmontese

38

Pinzgauer

33

40

Polled

33

36

Hereford

 

Red Poll

36

Sahiwal

38

Santa Gertrudis

33

Salers

35

38

Shorthorn

37

32

39

Simmental

39

43

40

South Devon

33

42

38

Tarentaise

33

38

NOTE: BIF, Beef Improvement Federation; AFRC, Agricultural and Food Research Council; MARC, Roman L.Hruska U.S. Meat Animal Research Center (USDA/ARS).

Sources: Beef Improvement Federation (1990), AFRC (1990), MARC, from data reported by Cundiff et al. (1988), and Gregory et al. (1982), which are from a particular sire breed on mature Angus and Hereford cows.

Gregory et al., 1992a). Weight of heifer calves average 7 percent less than bull calves at birth (Agricultural and Food Research Council, 1990; Beef Improvement Federation, 1990), and weight of calves born to 2-, 3-, and 4-year old cows average 8, 5, and 2 percent less than those born to



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Nutrient Requirements of Beef Cattle: Seventh Revised Edition, 1996 4 Reproduction GESTATION Meeting the nutrient requirements of pregnant female cattle is important to ensuring an adequate nutrient supply for proper growth and development of the fetus and to ensuring that the female is in adequate body condition to calve and lactate, to rebreed within 80 days after calving, and to provide, in the case of the 2- or 3-year-old heifer, adequate nutrients for continued growth. This section will concentrate on nutrient requirements for pregnancy—in particular, energy and protein—in cattle and some of the factors affecting those requirements. For lack of information to the contrary, it is generally assumed that nutrient needs for pregnancy are proportional to birth weight of the calf. Thus, it is assumed that factors that affect calf birth weight have a proportional affect on nutrient requirements during pregnancy. Factors known to affect calf birth weight include breed of sire, breed of dam, heterosis, parity of the dam, number of fetuses, sex of the fetus, environmental temperature, and nutrition of the dam (Ferrell, 1991a). Of the factors affecting calf birth weight, breed or genotype of the sire, dam, or calf generally has the greatest influence (Andersen and Plum, 1965). Typical birth weight of calves of various breeds are listed in Table 4–1. Birth weights of calves in one study differed by as much as 18 kg (Agricultural and Food Research Council, 1990). Ranges to 10 kg were reported for mean birth weights of calves of different breeds typically used for beef production in the United States (Beef Improvement Federation, 1990) or those of crossbred calves from Angus and Hereford dams (Gregory et al., 1982; Cundiff et al., 1988). Heterosis, resulting in increased birth weight, is generally about 6 to 7 percent when Bos taurus breeds are crossed, less (0 or negative) when Bos taurus sires are crossed on Bos indicus dams, but considerably higher (20 to 25 percent) when the reciprocal mating is made (Ellis et al., 1965; Long, 1980; TABLE 4–1 Estimated Birth Weight of Calves of Different Breeds or Breed Crosses, kg Breed BIF AFRC MARC Angus 31 26 35 Brahman 31 — 41 Braford 36 — — Brangus 33 — — Braunvieh — — 39 Charolais 39 43 40 Chianina — — 41 Devon 32 34 — Galloway — — 36 Gelbvieh 39 — 39 Hereford 36 35 37 Holstein — 43 — Jersey — 25 31 Limousin 37 38 39 Longhorn — — 33 Maine-Anjou 40 — 41 Nellore — — 40 Piedmontese — — 38 Pinzgauer 33 — 40 Polled 33 — 36 Hereford   Red Poll — — 36 Sahiwal — — 38 Santa Gertrudis 33 — — Salers 35 — 38 Shorthorn 37 32 39 Simmental 39 43 40 South Devon 33 42 38 Tarentaise 33 — 38 NOTE: BIF, Beef Improvement Federation; AFRC, Agricultural and Food Research Council; MARC, Roman L.Hruska U.S. Meat Animal Research Center (USDA/ARS). Sources: Beef Improvement Federation (1990), AFRC (1990), MARC, from data reported by Cundiff et al. (1988), and Gregory et al. (1982), which are from a particular sire breed on mature Angus and Hereford cows. Gregory et al., 1992a). Weight of heifer calves average 7 percent less than bull calves at birth (Agricultural and Food Research Council, 1990; Beef Improvement Federation, 1990), and weight of calves born to 2-, 3-, and 4-year old cows average 8, 5, and 2 percent less than those born to

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Nutrient Requirements of Beef Cattle: Seventh Revised Edition, 1996 5- to 10-year-old cows (Beef Improvement Federation, 1990; Gregory et al., 1990). Birth weight of calves born as twins is 25 percent less, but the total weight of twins average 150 percent of the birth weight of calves born as singles (Gregory et al., 1990). Severe energy or protein underfeeding has resulted in marked reductions of calf birth weight (Hight, 1966, 1968a,b; Tudor, 1972). Inadequate food intake during late pregnancy is also associated with weak labor, increased dystocia, reduced milk production and growth of progeny, and lowered rebreeding performance of the dam (Bellows and Short, 1978; Kroker and Cummins, 1979). Conversely, gross overfeeding during pregnancy can also result in reduced birth weight and subsequent decreased milk production, increased dystocia and neonatal death loss, and poor rebreeding performance (Arnett et al., 1971; Robinson, 1977). The relationship of calf birth weight to cow condition score is typified by data shown in Figure 4–1. Birth weight decreased as cow condition score decreased below 3.5 or increased above 7, but did not change within the range of cow condition scores of about 3.5 to 7. It is suggested that calf birth weight is not substantially influenced by cow nutritional status within a broad range, but may be reduced by extreme over- or underfeeding. In those situations, negative influences on rebreeding performance, dystocia, etc., are of greater concern than calf birth weight. Effects of Temperature Although this section is primarily concerned with factors affecting calf birth weight, it is important to note that high environmental temperature during or shortly after conception can significantly increase embryonic mortality in cattle as well as several other species (Bell, 1987). In addition, high environmental temperatures, particularly during early pregnancy, may result in a wide range of FIGURE 4–1 Relationship of calf birth weight to cow condition score in mature cows of nine breeds. congenital defects. Limited data are available from well-controlled studies of cattle to characterize the influence of elevated temperatures on calf birth weight (Collier et al., 1982) and, to this subcommittee’s knowledge, no data are available from controlled experiments to characterize influences of chronic cold exposure, although these effects have been well documented in sheep (Alexander and Williams, 1971; Rutter et al., 1971, 1972; Cartwright and Thwaites, 1976; Thompson et al., 1982; Bell, 1987). Numerous data are available, however, to indicate that calves born in the spring are heavier than those born in the fall (McCarter et al., 1991a), calves born in the northern areas of the United States are heavier than those born in southern areas, and that genotype/environment interactions may have important influences on calf birth weight (Burns et al., 1979; Olson et al., 1991). The magnitude of response of calf birth weight to environmental temperature is influenced by severity, duration, and timing of exposure as well as genotype of the dam. Factors Affecting Fetal Growth Considerable progress has been made toward understanding how various factors affect fetal growth and the ensuing birth weight. Normal fetal growth follows an exponential pattern (Figure 4–2). In cattle, weight of uterine and placental tissues also increase exponentially (Ferrell et al., 1976a; Prior and Laster, 1979). Growth and development of the uterus and placental tissues precedes fetal growth. Development of those tissues is required to support subsequent fetal growth (Ferrell, 1991b,c). Growth of the fetus is a result of its genetic potential for growth, which is reflected in its demand for nutrients and constraints imposed by the maternal and placental systems in meeting that demand (Gluckman and Liggins, 1984; Ferrell, 1989). The potential of the maternal and placental systems to meet those demands are reflected in uterine blood flow or placental size and functional capacity. The influence of maternal nutrition on fetal development is complicated by the fact that the fetus can be undernourished in well-fed mothers when placental size or function is inadequate to meet fetal demands. Conversely, even though the mother is undernourished, the maternal and placental systems may compensate such that fetal malnutrition is minimal (Bassett, 1986, 1991). Weight and perfusion of uterine and placental tissues are reduced with heat (Alexander and Williams, 1971; Cartwright and Thwaits, 1976; Reynolds et al., 1985; Bell et al., 1987) and with twins as compared with single fetuses (Bellows et al., 1990; Ferrell and Reynolds, 1992). These variables are also influenced by genotype of sire, dam, or fetus (Ferrell, 1991c). Numerous other data are available to indicate that perfusion of uterine and placental tissues and functional capacity of the placenta have central roles in fetal growth (Alexander, 1964a,b; Owens et al., 1986).

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Nutrient Requirements of Beef Cattle: Seventh Revised Edition, 1996 FIGURE 4–2 Relationship of fetal weight to day of gestation in cattle. The Role of the Placenta Functions of the placenta include exchange of metabolites, water, heat, and respiratory gasses. The placenta also serves as a site of synthesis and secretion of numerous hormones and extensive interconversion of nutrients and other metabolites (Munro et al., 1983; Battaglia, 1992). Placental transport of oxygen, glucose, amino acids, and urea and placental clearance of highly diffusible solutes increase during gestation as indicated by net fetal uptake or loss in both sheep and cattle (Bell et al., 1986; Reynolds et al., 1986). Because of the numerous metabolic functions of the uterus and placenta (uteroplacenta), oxidative metabolism is extensive throughout gestation. Even in late gestation when the fetus is several times larger than the placenta, energy consumption of the uteroplacenta is about equal to that of the fetus (Reynolds et al., 1986). Similarly, uteroplacental net use of glucose is at least 70 percent of gravid uterine glucose uptake, even in late gestation. Likewise, a major proportion of the net use of amino acids taken up from the uterine circulation is metabolized by the uteroplacenta (Reynolds et al., 1986; Ferrell, 1991b). An increase in maternal metabolism is also required to support the requirements of pregnancy. Thus, of the total increase in energy expenditure associated with pregnancy, about one-half may be attributed to metabolism of tissues of the gravid uterus and about one-fourth maybe attributed to the fetus per se (Kleiber, 1961; Ferrell and Reynolds, 1987). Energy Requirements Energy accretion in the gravid uterus of Hereford heifers bred to Hereford bulls has been reported by Ferrell et al. (1976a). The equation used to describe the relationship of energy content of the gravid uterus (Ye) vs day of gestation (t) in kcal, was Eq. 4–1 Similar values can be calculated from the data of Prior and Laster (1979) who used crossbred heifers bred to Brown Swiss bulls, and from the data of Jakobsen (1956) and Jakobsen et al. (1957) who used Red Danish cattle. Other information related to bovine fetal growth and weight change of the pregnant cow is available (Winters et al., 1942; Ellenberger et al., 1950; Eley et al., 1978; Silvey and Haycock, 1978). Eq. 4–1 was associated with a predicted calf birth weight of 38.5 kg. Scaling Eq. 4–1 by birth weight yields the following equation (kcal): Eq. 4–2 This equation may be differentiated with respect to t to estimate daily energy accretion in the tissues of the gravid uterus, yielding (kcal/day): Eq. 4–3 The gross efficiency of metabolizable energy (ME) use for accretion in the gravid uterus of cattle averaged 14 percent (Ferrell et al., 1976b). Other estimates with cattle and sheep average about 13 percent (Graham, 1964; Langlands and Southerland, 1968; Lodge and Heaney, 1970; Moe et al., 1970; Moe and Tyrrell, 1971; Sykes and Field, 1972; Rattray et al., 1974; Robinson et al., 1980). Some of the potential reasons for the low estimates of apparent gross efficiency have been discussed previously. Use of

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Nutrient Requirements of Beef Cattle: Seventh Revised Edition, 1996 the average value of 13 percent efficiency results in the following equation to estimate the daily ME requirement for pregnancy in cattle: Eq. 4–4 Some evidence is available to indicate that efficiencies of ME use for maintenance and pregnancy vary similarly (Robinson et al., 1980). Values for efficiency of utilization of ME for maintenance (km) may be calculated from the equation of Garrett (1980a) as follows: Eq. 4–5 or, where NEm is net energy required for maintenance. The estimate of ME required for pregnancy may be converted to NEm equivalent (kcal/day) by use of appropriate estimate of km as follows: Eq. 4–6 If it is assumed, for example, that cows typically consume primarily forage diets containing 2.0 Mcal ME/kg, km is expected to be 0.576. With this assumption, the NEm required for pregnancy may be estimated from the following equation (kcal/day): Eq. 4–7 Estimates of the NEm required for pregnancy, from this equation, are shown in Table 4–2. For comparison purposes, previous estimates from NRC (1984) and CSIRO (1990) are also shown. Protein Requirements Protein requirements for pregnancy may be estimated using the approach used with energy. Estimates of nitrogen (N) content of gravid uterine tissues at various stages of TABLE 4–2 Estimates of NEm (Mcal/day) Required for Pregnancy Days of Gestation This Report NRC, 1984 CSIRO, 1990 130 0.327 0.199 0.280 160 0.634 0.505 0.509 190 1.166 1.083 0.923 220 2.027 1.952 1.673 250 3.333 2.916 3.029 280 5.174 3.518 5.478 NOTE: Estimates are based on calf birth weight of 38.5 kg. gestation have been reported by Jakobsen (1956), Ferrell et al. (1976a), and Prior and Laster (1979). The equation derived by Ferrell et al. (1976a) to relate N (g) content of those tissues to day of gestation (t) was Eq. 4–8 As with energy, this relationship may be scaled by predicted calf birth weight (38.5 kg) to derive the following equation Eq. 4–9 Daily accretion of N in gravid uterine tissues may be calculated by differentiation of Eq. 3–9 with respect to t as follows: Eq. 4–10 Supplementary net protein required for pregnancy is estimated from daily N accretion in gravid uterine tissues as Eq. 4–11 Resulting values are shown in Table 4–3 for several stages of gestation. It should be noted that because of the high rate of metabolism of amino acids by uteroplacental and fetal tissues relative to accretion (Ferrell et al., 1983; Battaglia, 1992), as well as changes in extrareproductive tissue metabolism, these should be considered minimal estimates. LACTATION Milk production in the beef cow is difficult to assess. In contrast to the dairy cow, which is generally milked by machine two or more times daily, the beef cow is generally in a pasture or range environment and milk produced is consumed by the suckling calf. Numerous efforts have been made to assess milk production of beef cows with suckling calves with minimal disturbance of the normal routine of the cow and calf (Lampkin and Lampkin, 1960; Neville, 1962; Christian et al., 1965; Gleddie and Berg, 1968; Lamond et al., 1969; Deutscher and Whiteman, TABLE 4–3 Estimates of Available Net Protein Required for Pregnancy by Beef Cows on Several Days of Gestation Days of Gestation Available Protein, g/day 130 9.1 160 17.5 190 32.2 220 56.0 250 95.2 280 156.1 NOTE: Estimates are based on calf birth weight of 38.5 kg.

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Nutrient Requirements of Beef Cattle: Seventh Revised Edition, 1996 1971; Totusek et al., 1973). The primary methods include hand milking with the calf nursing, machine or hand milking after oxytocin injection, and weighing the calf before and after (weigh-suckle-weigh) nursing (Kropp et al., 1973; Totusek et al., 1973; Cundiff et al., 1974; Holloway et al., 1975; Neidhardt et al., 1979; Boggs et al., 1980; Gaskins and Anderson, 1980; Chenette and Frahm, 1981; Hansen et al., 1982; Butson and Berg, 1984a,b; Jenkins and Ferrell, 1984; Holloway et al., 1985; McMorris and Wilton, 1986; Daley et al., 1987; Clutter and Nielson, 1987; Beal et al., 1990; McCarter et al., 1991; Hohenboken et al., 1992; Jenkins and Ferrell, 1992). Estimates of milk yield of grazing cows have been made at intervals varying from daily to twice during the entire lactation period. Time of separation of the calf from the cow has varied from 4 to 16 hours. Under the above situations, milk yield estimates vary depending on the method used. In addition, milk yield estimates differ based on the genetic potential of the cow to produce milk, age and breed of the cow, capacity of the calf to consume milk (which is influenced by breed, size, age, and sex of the calf), nutritional status, thermal environment, and stage of lactation. The most commonly adapted procedure has been the weigh-suckle-weigh procedure, but several groups of researchers have used machine or hand milking. Many of the latter groups have reported composition of milk, as well as yield (Melton et al., 1967; Wilson et al., 1969; Kropp et al., 1973; Totusek et al., 1973; Cundiff et al., 1974; Holloway et al., 1975; Lowman et al., 1979; Rogers et al., 1979; Bowden, 1981; Chenette and Frahm, 1981; Mondragon et al., 1983; Butson and Berg, 1984a,b; McMorris and Wilton, 1986; Daley et al., 1987; Diaz et al., 1992; Masilo et al., 1992). It is important to note that composition as well as yield is variable. Some of the factors influencing milk composition include milk collection procedure, breed and age of cow, stage of lactation, and nutritional status. Whereas numerous reports have included measures of milk yield of beef cows or cows with suckling calves, the primary emphasis has been to assess relative yields for breed group comparisons or to estimate the relative influence of milk yield on calf preweaning growth (Drewry et al., 1959; Christian et al., 1965; Notter et al., 1978; Reynolds et al., 1978; Robinson et al., 1978; Williams et al., 1979; Bartle et al., 1984; Marshall et al., 1984; Miller and Deutscher, 1985; Fiss and Wilton, 1989; Montano-Bermudez et al., 1990; Green et al., 1991; Freking and Marshall, 1992; Gregory et al., 1992a,b). Only a limited number of studies have reported data from which the shape of the lactation curve can be assessed (Deutscher and Whiteman, 1971; Kropp et al., 1973; Totusek et al., 1973; Grainger and Wilhelm, 1979; Neidhardt et al., 1979; Gaskins and Anderson, 1980; Chenette and Frahm, 1981; Jenkins and Ferrell, 1984; Holloway et al., 1985; Jenkins et al., 1986; Clutter and Neilson, 1987; Sacco et al., 1987; Mezzadra et al., 1989; McCarter et al., 1991; Hohenboken et al., 1992; Jenkins and Ferrell, 1992) (Figure 4–3). These studies, unlike those with dairy cows, generally include a limited number of data points for a given cow during lactation, largely because of logistical problems described previously. The most widely applied equation for describing the lactation curve of dairy cattle has been that proposed and described by Wood (1967, 1969, 1976, 1979, 1980) of the form where the coefficients a, b, and c define the curve of production of a character Y at week n. Several other approaches have been proposed (Rowlands et al., 1982; FIGURE 4–3 Generalized lactation curves for cows producing 5, 8, 11, or 14 kilograms of milk at peak milk production.

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Nutrient Requirements of Beef Cattle: Seventh Revised Edition, 1996 Elston et al., 1989; Morant and Granaskthy, 1989) but, as with the Woods’ equation, their use with beef cattle milk production has been very limited because of the relatively large number of data points to fit the equation form. Jenkins and Ferrell (1984) proposed a similar equation form: Eq. 4–11 where Yn equals daily milk yield (kg/day) at week n postpartum, a and k are solution parameters, and e is the base of natural logarithms. This equation may be used to estimate the following values: Eq. 4–12 Eq. 4–13 Eq. 4–14 This equation form has been criticized (Hohenboken et al., 1992) but has an advantage over that of the Woods’ equation in that it can be fit with a minimal number of data points. In addition, curve parameters may be estimated from published data with a minimum of information. Available data (Deutscher and Whiteman, 1971; Jeffery et al., 1971; Kropp et al., 1973; Totusek et al., 1973; Grainger and Wilhelms, 1979; Neidhardt et al., 1979; Gaskins and Anderson, 1980; Chenette and Frahm, 1981; Jenkins and Ferrell, 1984; Holloway et al., 1985; Jenkins et al., 1986; Clutter and Neilson, 1987; Sacco et al., 1987; Lubritz et al., 1989; Mezzadra et al., 1989; McCarter et al., 1991; Hohenboken et al., 1992; Jenkins and Ferrell, 1992) indicate that peak lactation occurred at approximately 8.5 weeks postpartum in cows with suckling calves. Those data included a wide variety of breeds or breed crosses of cows, calves, milk yields, and sampling protocols. This value is somewhat later than generally observed for dairy cows and may reflect the influence of calf consumption capacity. Rearrangement of Eq. 4–12 yields Eq. 4–15 Maximum or peak yield of cows with suckling calves is variable, as noted above. Reported values range from about 4 to 20 kg/day. The highest values have been reported for Holstein or Friesian cows. More typically, reported values for dual purpose or dairy×beef crossbred cows have rarely exceeded 14 kg/day. Therefore, for the purposes of this publication, NEm and net protein requirements are for peak yield values of 5, 8, 11, and 14 kg/day for four types of cows typical of beef production enterprises (Tables 4–4 and 4–5). Rearrangement of Eq. 4–13 and solving for “a” yields estimates of 0.6257, 0.3911, 0.2844, and 0.2235 for cows having maximum yields of 5, 8, 11, and 14 kg/day at 8.5 weeks postpartum. Substitution of these values TABLE 4–4 Net Energy (NEm, Mcal/day) Required for Milk Production Week of Lactation Peak Milk Yield, kg/day 5 8 11 14 3 2.42 3.87 5.32 6.77 6 3.40 5.44 7.48 9.52 9 3.58 5.73 7.88 10.03 12 3.36 5.37 7.39 9.40 15 2.95 4.72 6.49 8.26 18 2.49 3.98 5.47 6.96 21 2.04 3.26 4.48 5.71 24 1.64 2.62 3.60 4.58 27 1.29 2.07 2.85 3.62 30 1.01 1.46 2.19 2.83 NOTE: Requirement assumes milk contains 4.0% fat, 3.4% protein, 8.3% SNF, and 0.72 Mcal/kg. TABLE 4–5 Net Protein (g/day) Required for Milk Production Week of Lactation Peak Milk Yield, kg/day 5 8 11 14 3 115 183 252 321 6 161 258 354 451 9 170 272 373 475 12 159 254 350 445 15 140 223 307 391 18 118 188 259 330 21 97 154 212 270 24 68 124 170 217 27 61 98 135 172 30 48 77 105 134 NOTE: Requirement assumes milk contains 3.4% protein. into Eq. 4–14 yields estimates of total milk yield over a 30-week lactation period of 701, 1,122, 1,543, and 1,963 kg. These values encompass nearly all reported values for total milk yield of beef cows with suckling calves. Expected maximum milk production is highly dependent on cow genotype and is about 26 and 12 percent lower for 2- and 3-year-old heifers, respectively, than for cows 4 years old or older (Gleddie and Berg, 1968; Gaskins and Anderson, 1980, Hansen et al., 1982; Butson and Berg, 1984a,b; Clutter and Nielson, 1987). Insufficient data are available to fully characterize the effects of age and breed of cow, stage of lactation, nutritional status, etc., on milk composition in beef cows. Therefore, for general purposes, mean of composition values for beef cows (Melton et al., 1967; Wilson et al., 1969; Kropp et al., 1973; Totusek et al., 1973; Cundiff et al., 1974; Holloway et al., 1975; Lowman et al., 1979; Bowden, 1981; Chenette and Frahm, 1981; Grainger et al., 1983; Mondragon et al., 1983; Butson and Berg, 1984a,b; McMorris and Wilson, 1986; Daley et al., 1987; Diaz et al., 1992; Masilo et al., 1992) is assumed. The average (mean±SD) value for milk fat was 4.03±1.24 percent (18 studies), for milk protein was 3.38±0.27 percent (10 studies), for solids

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Nutrient Requirements of Beef Cattle: Seventh Revised Edition, 1996 not fat (SNF) was 8.31±1.38 percent (10 studies), and for lactose was 4.75±0.91 percent (5 studies). Energy content (E, Mcal/kg) of milk may be calculated as follows (Tyrrell and Reid, 1965): Eq. 4–16 or Eq. 4–17 Committees of the National Research Council (1984, 1989) concluded that ME is utilized for lactation and maintenance with similar efficiencies; thus the energy content of the milk produced is equivalent to the NEm required for milk production (National Research Council, 1984). Data reported by Moe et al. (1970, 1972), Patle and Mudgal (1976), van der Honing (1980), Agricultural Research Council (1980), Garrett (1980b), Moe (1981), Munger (1991), Windisch et al. (1991), Gadeken et al. (1991), and Unsworth (1991), among others, support this conclusion. Although limited data are available, differences among breeds in efficiency of ME use for milk production appear to be minimal. BREEDING PERFORMANCE Beef cattle are managed under a wide variety of conditions. To a large extent their usefulness lies in their ability to harvest and utilize feed resources available under existing environmental conditions. The large variation in animal genotypes, environmental conditions, and available feed resources presents a challenge in determining and applying nutrient requirement guidelines. Providing nutrients to meet animal requirements is necessary for attainment of maximum production levels. However, it is frequently not economically advantageous to feed beef cattle in the breeding herd to meet their nutrient requirements throughout the year. Production levels to maximize net economic return vary based on interrelationships among numerous factors including, but not limited to, feed resources available, animal genotype, physiological state, costs of supplements, and environmental conditions. It should be recognized, however, that if the animals’ nutrient requirements are not met during part of the year, deficits must be made up during other parts of the year if production is to be maintained. In grazing, as in nongrazing situations, maximum efficiency of diet utilization is attained by providing nutritionally balanced diets. When energy is first limiting, for example, protein, minerals and vitamins are not efficiently utilized. Supplemental protein, in this case, will be used to meet energy needs until energy and protein are equally limiting. Conversely, if protein is first limiting, provision of additional energy will not improve performance and may in fact depress performance. These concepts are applicable to other nutrients as well, i.e., performance is limited to that which is supported by the first-limiting nutrient. In the grazing animal, the quantity and quality of forages are of primary concern because they provide the nutrient base. The most limiting nutrients are especially difficult to establish for grazing cattle because the quantity and quality of the diets selected by the animal are difficult to assess. This is of less concern when minimal variation in forage quality results in limited opportunity for selectivity, such as occurs most commonly during spring and winter grazing. The ultimate result of malnutrition of the beef herd is a reduction in the number of viable offspring produced. Influences of malnutrition are seen through effects on attainment of puberty, duration of the postpartum estrus, gametogenesis, conception rate, embryonic mortality, prenatal development, and sexual behavior. Some of these effects will be discussed briefly in subsequent sections. Readers are referred to recent reviews by Hurley and Doane (1989), Robinson (1990), Short et al. (1990), Ferrell (1991a), Dunn and Moss (1992), Schillo et al. (1992), and Patterson et al. (1992) for greater detail. Heifer Development Age at puberty is an important production trait in cattle because many of the currently used management systems require that heifers be bred, during a restricted breeding system, at 14- to 16-months-old to calve at 2 years old. Heifers that reach puberty early and have a number of estrous cycles prior to the breeding season have a higher conception rate and conceive earlier in the breeding season than ones that reach puberty later. In addition, heifers that conceive early in their first breeding season have a greater probability of weaning more and heavier calves during their productive lifetime. EFFECTS OF FEEDING Underfeeding, resulting in low growth rate of heifers, delays puberty in cattle; and the effects are more pronounced when applied in the early prenatal phase than when applied immediately prepubertal. In an extreme example, Rege et al. (1993) reported age at first calving in White Fulani cattle in Nigeria to be as late as 2,527 days (6.9 years). As an example of more typical conditions in temperate regions, Angus-Hereford crossbred heifers fed to gain 0.27, 0.45, or 0.68 kg/day reached puberty at an average age of 433, 411, and 388 days old, respectively (Short and Bellows, 1971). Although these differences are relatively small, pregnancy rates after a 60-day breeding season were 50, 86, and 87 percent, respectively.

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Nutrient Requirements of Beef Cattle: Seventh Revised Edition, 1996 EFFECTS OF MATURITY Both age and weight at puberty differ substantially among breeds of cattle (Laster et al., 1972, 1976, 1979; Stewart et al., 1980; Sacco et al., 1987). Within beef breeds, those having larger mature size tend to reach puberty at a later age and heavier weight. Bos indicus heifers tend to reach puberty at an older age than Bos taurus heifers, and heifers from higher milk-producing breeds are generally younger at puberty than those from breeds having lower milk production. Some of those differences are likely the result of direct maternal effects expressed through higher rates of preweaning gain by calves from higher milk-producing breeds. Numerous data are available that indicate that neither age nor weight is a reliable indicator of reproductive development but that threshold values for both age and weight must be reached before puberty can occur. This conclusion is similar to the “physiological maturity” concept proposed by Joubert (1963) and to the “target weight” concept proposed by Lamond (1970). These concepts have been used by Spitzer et al. (1975), Dziuk and Bellows (1983), and Wiltbank et al. (1985) to suggest that replacement heifers should be fed to reach a preselected or “target” weight at a given age. Heifers of most Bos taurus breeds of cattle are expected to reach puberty by 14 months old or younger, if fed adequately. However, threshold ages of some heifers of Bos indicus breeds may be older than 14 months. Generally, heifers of typical Bos taurus beef breeds (e.g., Angus, Charolais, Hereford, Limousin) are expected to reach puberty at about 60 percent of mature weight. Heifers of dual purpose or dairy breeds (e.g., Braunvieh, Brown Swiss, Friesian, Gelbvieh, Red Poll) tend to reach puberty at a younger age and lower weight, relative to mature weight (about 55 percent of mature weight) than those of beef breeds. Conversely, heifers of Bos indicus breeds (e.g., Brahman, Nellore, Sahiwal) generally reach puberty at older ages and heavier weights (about 65 percent of mature weight) than those of Bos taurus beef breeds (Laster et al., 1972, 1976, 1979; Stewart et al., 1980; Ferrell, 1982; Sacco et al., 1987; Martin et al., 1992; Gregory et al., 1992b; Vera et al., 1993). Mature weight refers to weight reached at maturity by cows of the same genotype in a nonrestrictive environment (for example, mature weight as determined by genetic potential). In a restrictive environment (high environmental temperature, limited nutrition, parasite loads, etc.), mature weight of cows is often less than that of cows of similar genotype maintained in a less restrictive environment (Butts et al., 1971; Pahnish et al., 1983). Heifer weight at puberty is also reduced, but to a lesser extent than is mature weight. Thus, under those types of conditions, weight at puberty is generally a greater percentage of observed mature weight than described above (Vera et al., 1993). If the target weight and age to reach puberty are established, and present age and weight are known, rates of gain needed to achieve the target weight and age can easily be calculated. Energy and protein needs to meet those rates of gain can be estimated by use of the previously described net energy and net protein equations for growing heifers. Excessive feeding should be avoided. In addition to increasing feed costs, overfeeding that results in excess fat accretion may have detrimental effects on expression of behavioral estrous, conception rate, embryonic and neonatal survival, calving ease, milk production, and productive life. Weight and Condition Changes in Reproducing Females Composition of weight change in growing and mature cattle has been discussed in other sections and will not be discussed in detail here. In the mature cow, weight change, with the exception of weight change associated with pregnancy or parturition, primarily reflects change in body condition. In the developing heifer, percentage of body fat and body condition may decrease, even though weight may continue to increase because of skeletal and muscle growth at the expense of body fat. In both the heifer and cow, weight gain associated with pregnancy and weight loss at parturition should not be construed as change in maternal weight or condition. Weight gain during pregnancy and loss at parturition is about 1.7 times calf birth weight and represents weight gain or loss of the fetus, fetal fluids, placenta, and uterus. For many practical purposes, subjective evaluation of body fatness by use of a visual condition scoring system (1=thinnest, 9=fattest) is frequently of benefit. More accurate methods are available for measuring body composition, but their use is generally limited to experimentation because of high costs or amount of labor required. Death of calves perinatally represents a major production loss for beef cattle. Neonatal mortality is related to birth weight with the greatest losses occurring at low and high birth weights and lower mortality associated with moderate birth weights. Because dystocia, which is positively associated with birth weight, is a major cause of neonatal calf death (Laster and Gregory, 1973; Bellows et al., 1987), some cattle producers have attempted to reduce calf birth weight, particularly in first calf heifers, by underfeeding during the last trimester of pregnancy. As noted previously, malnutrition must be relatively severe to result in substantial reductions in calf birth weight. In nine studies reviewed by Dunn (1980), birth weight was reduced in all but one by severe underfeeding, but dystocia was reduced in only one (Dunn and Moss, 1992); but by underfeeding sufficiently to reduce birth weight, calf survival was reduced. In addition, numerous data (Short et al., 1990; Ferrell, 1991; Dunn and Moss, 1992) indicate the interval from

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Nutrient Requirements of Beef Cattle: Seventh Revised Edition, 1996 calving to rebreeding is increased by underfeeding during late pregnancy. Inadequate prepartum nutrition is also associated with lower milk production and decreased calf weight at weaning (Might, 1968; Corah et al., 1975; Bellows and Short, 1978). Negative effects of underfeeding during pregnancy are more severe in first-calf heifers than in more mature cows. The interval from calving until conception cannot exceed approximately 80 days if an annual calving interval is to be maintained in beef cows. To have a high probability of conception by 80 days postcalving, the interval of postpartum anestrous should be 60 days or less. For this reason, numerous researchers have studied the period of postpartum anestrous (see Short et al., 1990). The duration of the postpartum anestrous period is increased in cows fed low concentrations of energy during late gestation or early lactation. However, response to low energy intake prepartum or weight change prepartum depends on body condition at calving. Cows that are in good body condition at calving (condition score =5) are minimally affected by either pre-or postpartum weight changes. Postpartum anestrous interval is increased by weight loss in cows that are in thin-to-moderate body condition (condition score =4) prior to calving. This problem is exacerbated by insufficient energy intake and weight loss postpartum. Effects of poor body condition at calving can be partly overcome by increased postpartum feeding. However, the postpartum period is a period of high metabolic demand because of the high nutrient requirements during early lactation. Thus, it is difficult to feed enough energy to cows during the early postpartum period to compensate for poor body condition at calving. This problem is intensified in heifers because of the additional nutrient needs for growth during the lactational period. Conversely, cows that are obese at calving have greater incidence of metabolic, infectious, digestive, and reproductive disorders than cows in moderate-to-good body condition. The duration of the postpartum interval of anestrous is longer in suckled than in milked or nonlactating cows. The delay in initiation of estrous cycles postpartum appears to result primarily from calf contact rather than suckling or lactation per se. In addition, the calf stimulus interacts with the nutritional status of the cow such that postpartum interval of anestrous is increased to a greater extent in cows in poor body condition than in those in good condition. Early weaning of calves, short-term weaning, or partial weaning, such as once per day suckling, have reduced the postpartum interval in anestrous beef cows, but successful use of any of these approaches requires intensive management and other inputs. It should be most efficient biologically to maintain cows in good condition throughout the year because of inefficiencies involved in depletion and repletion of body tissues. In addition, cows in good body condition are more tolerant of cold and other stresses. However, in many production situations, cows lose weight during early lactation when feed quantity is limited and quality is low and gain weight when higher quality feeds are more abundant or when nutrient demands are less. This cyclic loss and gain, although biologically less efficient, may be more efficient economically and may not be detrimental to total production, depending on the duration and severity of poor-feed conditions and the physiological status of the animals. Males Nutrient requirements for normal growth of young bulls have been discussed in previous sections, and estimates of requirements for maintenance and growth have been indicated. Details about nutritional influences on sexual development of young bulls as well as influences on sexual behavior, mating ability, and semen quantity and quality have been discussed in greater detail in reviews cited earlier in this section; thus it will be discussed only briefly here. Nutrient intakes below requirements result in reduced growth rates and delayed puberty in the male, as in the female and, if severe enough, can permanently impair sperm output (Bratton et al., 1959; VanDemark et al., 1964; Nolan et al., 1990). Inadequate nutrient intake is associated with reduced testicular weight, secretory output of the accessory sex glands, sperm motility and sperm concentration. Similarly, the reproductive potential of young males may also be impaired by overfeeding (Coulter and Kozub, 1984). Overfeeding has been associated with decreased scrotal circumference, epididymal sperm reserves, and seminal quality; however, it appears to be more likely to underfeed, particularly bulls of large breeds, than to overfeed (Pruitt and Corah, 1985). Negative influences of specific nutrient deficiencies have been discussed in detail by Hurley and Doane (1989). Mating behavior is an important aspect of male reproductive function as it has a direct bearing on the number of females mated. Moderate energy or protein deficiency or excess seem to have little effect on mating behavior, spermatogenesis, or semen quality. Severe deficiencies may result in diminished libido, depression of endocrine testicular function, and arrest of growth and secretory activity of accessory sex glands. Prolonged severe malnutrition, particularly insufficient intake of energy, protein, or water can lead to reduction or cessation of spermatogenesis and a reduction in semen quality. These effects are accompanied by decreased size of the testes and accessory sex glands. Atrophy of the interstitial and Sertoli cell populations may accompany these changes. Nevertheless, overall, it is evident that unless males are severely deprived, there is minimal effect on the sexual responses and efficiency of the mating responses. Conversely, overfeeding and obesity may result in diminished sexual activity. Overly fat males

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Nutrient Requirements of Beef Cattle: Seventh Revised Edition, 1996 may become less willing and able to inseminate females. Specific nutrient deficiencies may result in lowered physical ability to mate in addition to specific effects noted by Hurley and Doane (1989). It should be noted that the negative effects of malnutrition are more evident in the young male than in older animals. The mature male is remarkably resistant to nutritional stress, and infertility problems of nutritional origin are not often encountered. Both young and mature males frequently lose weight during the breeding season resulting from both decreased food consumption and substantially increased physical activity. Thus, bulls should be in good body condition at the beginning of the breeding season to provide energy and protein reserves for use during breeding. REFERENCES Agricultural and Food Research Council. 1990. Nutrient Requirements of Ruminant Animals: Energy. Nutr. Abstr. Rev. Ser. B 60:729–804. 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