have a feedback role in controlling feed intake (National Research Council, 1987). Regardless of the mechanism, the percentage of body fat is often considered in equations to predict feed intake by beef cattle. Fox et al. (1988) suggested that DMI decreases 2.7 percent for each 1 percent increase in body fat over the range of 21.3 to 31.5 percent body fat. As a result of the relationship between feed intake and body fat, careful monitoring of feed intake can be a useful management tool to determine when cattle have reached appropriate slaughter condition.

Sex (steer vs heifer) seems to have limited effects on feed intake (Agricultural Research Council, 1980; National Research Council, 1987). Intake differences attributable to sex may be evident at certain times; Ingvartsen et al. (1992a) reported that at body weights (BW) less than 250 kg, heifers had greater intake capacity than steers or bulls. In the previous edition of Nutrient Requirements of Beef Cattle (National Research Council, 1984), the Subcommittee on Beef Cattle Nutrition suggested that predicted DMI should be decreased by 10 percent for medium-framed heifers. At a given BW, heifers are proportionally more mature (fatter) than steers; hence, Fox et al. (1988) in their equation for predicting DMI use a frame-equivalent weight adjustment instead of a direct adjustment for sex.

The age of an animal when it is placed on feed can affect feed intake. Older animals (e.g., yearlings vs calves) typically consume more feed per unit BW than younger ones. Presumably, the greater ratio of age to body weight (age relative to proportion of mature body composition) for yearling cattle prompts greater feed intake. This effect has been likened to increased feed intake by cattle experiencing compensatory growth (National Research Council, 1987). Assuming that cattle started on feed at heavier BW are generally older cattle, age-related effects on feed intake are partly responsible for the positive relationship between initial weight on feed and DMI (National Research Council, 1987). The 1984 subcommittee (National Research Council, 1984) and Fox et al. (1988) suggested a 10 percent increase in predicted DMI by cattle started on feed as yearlings compared with cattle started on feed as calves. Before more accurate predictions of feed intake are possible, designed studies are needed in which independent effects of age and body weight or body composition on feed intake can be quantified.

The animal’s physiological state can markedly alter feed intake. Lactating animals can increase feed intake by 35 to 50 percent compared with that of nonlactating animals of the same BW fed the same diet (Agricultural Research Council, 1980). For forages, Minson (1990) reported a mean increase in DMI of 30 percent during lactation. Based on data from dairy cows, the Agricultural Research Council (ARC) (1980) and National Research Council (NRC) (1987) reports suggested that DMI increases by 0.2 kg/kg fat-corrected milk. Hence, beef cows bred for greater milk-producing ability would be expected to have greater feed intakes per unit BW during lactation. Advancing pregnancy has an adverse affect on feed intake, most notably during the last month (Agricultural Research Council, 1980; National Research Council, 1987). Ingvartsen et al. (1992a) noted a 1.5 percent decrease per week during the last 14 weeks of pregnancy in Danish Black and White heifers fed diets predominantly of roughage; this value agrees fairly well with the decrease of 2 percent per week during the last month of pregnancy suggested in the NRC (1987) report.

Frame size varies considerably in beef cattle. The 1984 NRC subcommittee (National Research Council, 1984) factored frame size into intake predictions, whereas Fox et al. (1988) suggested predictions could be adjusted by scaling frame sizes to an equivalent mature weight (frame-equivalent weight). However, Holstein and Holstein×beef crosses may consume more feed relative to body weight than beef breeds (National Research Council, 1987). Fox et al. (1988) suggested that intake predictions should be increased 8 percent for Holsteins and 4 percent for Holstein×British breed crosses relative to British-breed cattle. In addition to possible breed-specific effects, in the NRC (1987) report it was noted that genetic selection for feed efficiency could produce animals with increased feed intake potential, suggesting that genetic potential for growth (or increased production demands) may affect feed intake.

Environmental Factors Affecting Feed Intake

Considerable research has been conducted to evaluate effects of ambient temperature on feed intake and digestive function, and the topic has been reviewed extensively (Kennedy et al., 1986; Minton, 1986; Young, 1986; Young et al., 1989). In experimental situations, feed intake has been shown to increase as the temperature falls below the thermoneutral zone and decrease above that zone. With cold stress, ruminal motility and digesta passage increase before changes in intake occur, prompting Kennedy et al. (1986) to conclude that the digestive tract response may be essential to accommodating greater feed intake. As noted by Young (1986), however, this general response to temperature can vary with thermal susceptibility of the animal, acclimation, and diet. Behavioral responses to thermal stress (e.g., decreased grazing time) are restricted by some experimental conditions that could heighten the effects of thermal stress on feed intake. For example, acute cold stress decreased forage intake by as much as 47 percent in grazing cattle (Adams, 1987); however, for thermally adapted grazing cows, Beverlin et al. (1989) reported only small changes in forage intake with temperature deviations of 8° to -16° C. Similarly, feed intake by confined beef cattle fed finishing diets did not generally increase during

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