Abstract
A mathematical analysis of heat production by growing ruminants was used to assess the physiological importance of assimilation and digestion of dry matter intake. Energy retention (ER) was calculated according to National Academy of Sciences-National Research Council (NAS-NRC) (1984) for 300- and 600-kg cattle fed hay or corn silage with approximately 0, 25, 50 and 75% oat or corn grain. Protein synthesis was calculated for each ER using NAS-NRC (1984) standards for medium- and large-frame bulls, steers and heifers. Fat retention was then estimated by removing the caloric value of protein from ER and dividing the result by the caloric value of fat. Heat production (Y, MJ), obtained as the difference between metabolizable energy (ME) intake and energy retention, was related to the animal's metabolic weight (X1, kg), to the protein (X2, kg) and fat (X3, kg deposited and to the dry matter intake (X4, kg):[Formula: see text]This equation explained 99.9% of the variation of individual heat productions predicted by the California net energy system (NAS-NRC 1984). Heat production per kilogram dry matter intake (3.85 MJ) ranged from 38% at maintenance to 48% above maintenance of the total heat produced, which is similar to values reported in the literature from physiological studies. Metabolizable energy efficiency for fasting, gain of protein and gain of fat was respectively 102, 63 and 64%, whatever feed-stuffs were used. The proposed energy system can be summarized and used as follows: ME requirement = ME for fasting + ME for gain of fat + ME for gain of protein + ME for dry matter intake. In this form, ordinary ME values for feedstuffs are used. Key words: Ruminant, growth, protein fat efficiency, system, intake energy
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