The influence of energy density on the performance of feedlot cattle.
This study examined the interaction of diets differing in their energy densities and heat increments of feeding on the feed intake patterns, physiological measurements, empty body composition, and animal performance of steers in a feedlot environment. The energy densities of the diets ranged from 7.97 to 11.83 MJ ME and 6.50 to 9.53 MJ Effective Energy (BE) and the ratio of EE to ME ranged from 0.79 to 0.84. The feed intake pattern of steers was not affected by differences in the diets energy densities but was affected by diets that differed in their heat increments of feeding. The physiological measurements, rectal temperatures measured at 9.00 am and 2.00 pm (TR 9.00 am and 2.00 pm) and respiration rates of steers in the feedlot were compared to control steers kept on pasture. Steers in the feedlot registered significantly (P < 0.001) higher physiological measurements than the controls and the accepted norms for cattle not under heat stress. A relationship exists between the pattern of physiological measurements over time and feed intake pattens over time. Physiological measurements peak and dip during the same weeks as the feed intakes peak and fall. Peaks and the immediate dips thereafter are related to points of acute response resulting in a chronic response and acclimatisation. All feedlotted steers experienced heat stress within the first week of feeding. Steers feedlotted in summer took 28 days to achieve their peak feed intake whereas steers feedlotted in winter required 42 days to reach their peak feed intake. Steers that required 42 days in which to reach their peak intakes had greater increases in their daily intakes than those that required 28 days to reach their peak intakes. Steers feedlotted in winter lost their winter coat between weeks three and six. Differences in peak feed intakes were recorded for animals of a heavier starting live weight (late versus early maturing and long yearling versus weaners). Peak feed intake increased in line with increasing live weight at the start of feedlotting. These differences were attributed to their greater surface area and hence greater heat loss capacity. Comparison of steers tissue deposition rates of steers on diets differing in their ratio of EE to ME revealed non significant differences in the growth rates of protein and lipid. The proportional use of energy intake was significantly different with significantly (P < 0.1) more of the daily energy intake being utilised for lipid deposition in diets with a higher heat load. Animals suffering from differing heat loads were inhibited in depositing protein but were able to deposit lipid due to the associated lower heat production. This enforced deposition of lipid results in animals reaching slaughter condition after similar lengths of time but at lower ADG and lower carcass weights. The economic consequences are that the returns are higher due to higher carcass gains for steers fed diets with a higher ratio of EE to ME.
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