Owens F N, Gill D R, Secrist D S, Coleman S W
Oklahoma Agricultural Experiment Station, Animal Science Department, Stillwater 74078, USA.
J Anim Sci. 1995 Oct;73(10):3152-72. doi: 10.2527/1995.73103152x.
Growth in animals is defined as accretion of protein, fat and bone. Although growth typically is measured as the change in live weight, nutrient retention is estimated more precisely by measuring empty body weight and composition, whereas production economics are measured ideally through carcass weights and quality. As a percentage of live weight gain, carcass weight gain usually is a much higher percentage during the feedlot phase than during the growing phase of production because dressing percentage (ratio of carcass:live weight) increases with maturation and is greater with concentrate than with roughage diets. At a given fraction of mature body size (maximum body protein mass), body fat percentage seems to be a constant. Mature size may be altered genetically and nutritionally. Protein accretion declines to zero when cattle reach their mature body size (approximately 36% fat in empty body weight in modern cattle) even though mature animals can continue to accrete fat. Although fat accretion can be reduced by limiting the supply of net energy, rate of fat accretion by finishing steers given ad libitum access to high-concentrate diets seems to reach a plateau at approximately 550 g daily. Protein mass, in contrast, increases in proportion to empty body weight. The protein:fat ratio of the carcass can be increased through increasing mature size, by administering hormones or hormonal modifiers, by limiting energy intake during the growing period or finishing period, or by slaughtering cattle at an earlier stage of maturity. Energetically, efficiency of accretion of fat is approximately 1.7 times that of protein. But because more water is stored with deposited protein than with deposited fat, lean tissue gain is four times as efficient as accretion of fat tissue. Conversion of protein to fat is very inefficient, suggesting that excess protein is utilized inefficiently.
动物的生长被定义为蛋白质、脂肪和骨骼的增加。虽然生长通常以活体重的变化来衡量，但通过测量空腹体重和组成能更精确地估算营养物质的保留量，而生产经济学理想情况下是通过胴体重和品质来衡量的。作为活体重增加的百分比，在育肥阶段胴体重增加的百分比通常比生长阶段高得多，因为屠宰率（胴体：活体重的比率）随着成熟度增加，且与精料日粮相比，粗料日粮的屠宰率更高。在成熟体尺（最大体蛋白质量）的给定比例下，体脂百分比似乎是一个常数。成熟体尺可通过遗传和营养方式改变。当牛达到其成熟体尺时（现代牛空腹体重中约含36%的脂肪），蛋白质的增加量降至零，尽管成熟动物仍可继续积累脂肪。虽然通过限制净能量供应可以减少脂肪的积累，但自由采食高浓度日粮的育肥牛的脂肪积累速率似乎在每天约550克时达到平台期。相比之下，蛋白质量与空腹体重成比例增加。胴体的蛋白质：脂肪比率可通过增加成熟体尺、施用激素或激素调节剂、在生长阶段或育肥阶段限制能量摄入或在成熟早期屠宰牛来提高。从能量角度来看，脂肪积累的效率约为蛋白质的1.7倍。但由于储存于沉积蛋白质中的水分比沉积脂肪中的多，瘦肉组织的增加效率是脂肪组织积累效率的四倍。蛋白质向脂肪的转化效率非常低，这表明过量的蛋白质利用效率低下。
