cold stress and was often less during winter than during other seasons (Stanton, 1995).
Other adverse environmental conditions (wind, precipitation, mud, and so on) can accentuate the effects of ambient temperature. Fox et al. (1988) suggested multiplicative correction factors to adjust intake predictions for various environmental effects. Duration of adverse conditions seems important, and because effects caused by environmental conditions are variable, feed intake in a variable environment is difficult to predict (National Research Council, 1987). Regardless of the variable nature of its effects, thermal stress can markedly alter energetic efficiency of ruminants as evidenced by the effects of cold stress on energy utilization by beef cattle (Delfino and Mathison, 1991).
Seasonal or photoperiod (day length) effects on feed intake are understood less fully than are thermal effects, and photoperiod has been suggested as a potentially important factor influencing feed intake by beef cattle (National Research Council, 1987). Ingvartsen et al. (1992b) evaluated effects of day length on voluntary DMI capacity of Danish Black and White bulls, steers, and heifers. Voluntary DMI increased 0.32 percent per hour increase in day length; the range in the literature reviewed by the authors was -0.6 to 1.5 percent. Based on the deviation from the voluntary intake at 12 hours of daylight, voluntary intake would be expected to be 1.5 to 2 percent greater in long-day months (July in the northern hemisphere) and 1.5 to 2 percent less in short-day months (January). Hicks et al. (1990) grouped intake data into four seasons and thereby accounted for much of the seasonal pattern in feed intake. Nevertheless, temperature, photoperiod, animal, and perhaps management differences contribute to seasonal patterns, and separate effects are difficult to delineate.
With grazing cattle, quantity of forage available can affect feed intake. The authors of the NRC (1987) report reviewed data summarized by Rayburn (1986) and concluded that grazed forage intake was maximized when forage availability was approximately 2,250 kg dry matter/ha or a forage allowance of 40 g organic matter/kg BW. Intake decreased rapidly to 60 percent of maximum when forage allowance was 20 g organic matter/kg BW (450 kg/ha; National Research Council, 1987). Minson (1990) noted that bite size decreased with forage mass of less than 2,000 kg dry matter/ha; this decrease was only partially compensated for by increased grazing time, resulting in decreased forage intake. The break point at which intake of grazed forage was decreased with decreasing forage allowance seemed to lie between 30 and 50 g dry matter/kg BW. Relationships may vary with forage type and sward structure. McCollum et al. (1992) evaluated effects of forage availability on cattle grazing annual winter wheat pasture and noted that peak intake of digestible organic matter was predicted at 1,247 kg dry matter/ha or an allowance of approximately 300 g dry matter/kg BW. The data base for determining the relationship between forage availability and forage intake is derived largely from studies with actively growing pastures. As noted by Minson (1990), gain by sheep is related more closely to green (growing) forage allowance than to total forage dry matter offered. Similarly, Bird et al. (1989) reported that body weight gain by grazing cattle could be modeled more effectively from green pasture mass than from total pasture mass. Selective grazing of growing forage may increase in pastures with both growing and senescent material. Cattle eat only small amounts of senescent forage when some growing forage is available (Minson, 1990). Hence, effects of forage availability on intake should be considered in light of pasture composition and the potential for selective grazing.
Growth-promoting implants tend to increase feed intake. In two trials with beef steers fed a 60 percent concentrate diet, administering an estradiol benzoate/progesterone implant increased DMI from 4 to 16 percent, depending on when the implant was administered relative to slaughter (Rumsey et al., 1992). Fox et al. (1988) suggested that predicted feed intake should be decreased 8 percent for nonimplanted cattle.
Monensin, the ionophore feed additive, typically decreases feed intake. Fox et al. (1988) suggested that feed intake decreases by 10 and 6 percent with 33 and 22 mg monensin/kg diet respectively. With beef steers fed a 90 percent concentrate diet, Galyean et al. (1992) noted a 4 percent decrease in feed intake when animals were fed 31 mg monensin/kg dietary dry matter. Lasalocid, another ionophore approved for use in beef cattle, seems to have limited effects on feed intake. Fox et al. (1988) suggested that feed intake is decreased 2 percent by lasalocid, regardless of dietary concentration. Malcolm et al. (1992) found that feed intake increased approximately 4 percent with 85 percent concentrate diets that contained 33 mg lasalocid/kg diet compared with a nonionophore, control diet. Fewer data are available regarding effects of laidlomycin propionate, an ionophore approved for confined growing and finishing cattle, on feed intake. However, a summary of available data (Vogel, 1995) suggests that laidlomycin propionate has minimal effect on feed intake.
A dietary nutrient deficiency, particularly protein, can decrease feed intake. With low-nitrogen, high-fiber forage, nitrogen deficiency is common, and provision of supplemental nitrogen often increases DMI substantially (Galyean and Goetsch, 1993). Forage intake responses to protein are most typical when forage crude protein content is less than 6 to 8 percent (National Research Council, 1987). Supplementing forages with grain-based concentrates often decreases forage intake, such effects typically being