奶牛小肠蛋白质体系的局限性与氨基酸平衡

现在国内外已将奶牛的可消化粗蛋白体系改进为小肠可消化蛋白质体系,但由于小肠蛋白质的氨基酸组成变异性很大,其CV高达5.6%~19.2%,因而导致可代谢蛋白质的转化效率不稳定,表观效率便会掩盖实际需要。奶牛的第一和第二限制性氨基酸分别为赖氨酸和蛋氨酸,较多研究认为组氨酸为第三限制性氨基酸,但当肝后部提供的组氨酸不能满足产奶需要时,组氨酸可来自体储存的分解,因此尚不能完全确定。据对87个试验结果的统计分析表明,在瘤胃后供给额外的赖氨酸和蛋氨酸,则奶牛的产奶量、乳蛋白量、乳蛋白率均明显增加;对48个试验结果的统计分析结果表明,在可代谢蛋白质被满足供应的基础上,当赖氨酸和蛋氨酸分别达到7.08%和2.35%时,乳蛋白质产量达到最高点。法国INRA在其试验的基础上,确定小肠可消化真蛋白质中的赖氨酸和蛋氨酸的适宜含量为7.3%和2.5%,由于我国现行的是小肠可消化粗蛋白质体系,故建议其赖氨酸和蛋氨酸的含量为6.0%和1.9%,但须评定瘤胃微生物蛋白质和饲料非降解蛋白质的赖氨酸和蛋氨酸含量,并调整日粮使其达到平衡。

The Limitation of Digestible Protein in the Small Intestine of Dairy Cattle and Its Balance of Amino Acid

The protein value of feeds for dairy cattle has been assessed by use of the digestible crude protein or true protein in the small intestine.The protein system for dairy cattle in China is known as digestible crude protein in the small intestine(DCPSI),comprising microbial digestible protein and undegradable protein. Studies have demonstrated that there is great variability in the composition of intestinal digesta amino acid along with the variability in the composition of undegradable feed protein and amino acids which limits the accuracy of protein value and the prediction of milk production.The balance of amino acids in the small intestine should be included.Generally recognize that Lys and Met are the first and second-limiting essential amino acids in protein for dairy cattle,the following limiting EAA has not been decided yet.Thus we proposed the proportion of Lys and Met in the DCPSI.The proportion of Lys and Met in microbial protein averages 7.4% and 2.09%,respectively.Results of experiments showed that the degradability of Lys and Met of concentrates were not significantly different from the crude protein degradability in the rumen,thus the calculation could be simplified.It was proposed that the proportion of Lys and Met in the DCPSI were 6.0% and 1.9% for feeding practice,respectively.