Growth and development with special reference to domestic animals XXXV : energetic efficiency of milk production and the influence of body weight thereon
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Analyses of records for feed consumption, milk production, and body weight changes of 243 mature cows (5 years or over) showed that 0.305 pound TDN (total digestible nutrients) is required to produce 1 pound FCM (4% milk); 2.1 pounds TDN are required to gain 1 pound, live weight; 0.053 pound TDN is required to maintain 1 pound live weight at body weight lIb ifit is assumed that maintenance cost increases not with simple body weight, but with body weight raised to the 0.73 power (as was found for basal metabolism and endogenous nitrogen). From these results the following conclusions were deduced: (A) The net digestible feed energy cost of milk production (not counting, maintenance cost or live-weight gain cost) is about 1.6 times the milk energy; or the net (or partial) energetic efficiency (ratio of milk energy to digestible feed energy less maintenance energy) is about 60%. (B).The gross digestible feed energy cost of milk production (including maintenance cost) is about 3 times the milk energy; or the gross (or overall, or total) energetic efficiency of milk production (ratio of milk energy to total digestible feed eriergy) is about 30% (exact value depending on milk yield). (C) The digestible energy cost of maintenance is about 2.4 times the basal (energy) metabolism. All these conclusions are but rough approximations to the true values because the basic da ta are not homogeneous. Gross efficiency of milk production in the given group of cattle declined with increasing live weight, thereby confirming Gaines'conclusion. However, while this decline in efficiency with increasing body weight is significant statistically, it may not be significant physiologically; available evidence indicates that larger cows tend to be fed somewhat more liberally than smaller cows, and that efficiency tends to decrease with increasing plane of nutrition.