Department of Animal Science, University of Tennessee, Knoxville 37996.
Department of Animal Science, University of Tennessee, Knoxville 37996.
J Dairy Sci. 2017 Oct;100(10):8132-8145. doi: 10.3168/jds.2017-13026. Epub 2017 Jul 26.
The objective of this study was to examine the effect of reducing rumen-degradable protein (RDP) and rumen-undegradable protein (RUP) proportions on feed intake, milk production, and N-use efficiency in primiparous and multiparous cows exposed to warm climates. Eighteen primiparous and 30 multiparous mid-lactation Holstein cows were used in a completely randomized design with a 2 × 2 factorial arrangement of treatments. Cows were randomly assigned to 1 of 4 dietary treatments formulated to contain 2 proportions of RDP (10 and 8%) and 2 proportions RUP (8 and 6%) of dry matter (DM) indicated as follows: (1) 10% RDP, 8% RUP; (2) 8% RDP, 8% RUP; (3) 10% RDP, 6% RUP; and (4) 8% RDP, 6% RUP. Protein sources were manipulated to obtain desired RDP and RUP proportions. Diets were isoenergetic and contained 50% forage and 50% concentrate (DM basis). Cows were individually fed the 10% RDP, 8% RUP diet 3 wk before treatment allocation. Cows were exposed to the prevailing Tennessee July and August temperature and humidity in a freestall barn with no supplemental cooling. Main effects and their interaction were tested using the Mixed procedure of SAS (least squares means ± standard error of the mean; SAS Institute Inc., Cary, NC). Observed values of nutrient intake and milk production were used to obtain NRC (2001) model predictions. Cows showed signs of heat stress throughout the study. Reducing from 10 to 8% RDP decreased dry matter intake (DMI; 0.9 kg/d) at 8% RUP, but increased DMI (2.6 kg/d) at 6% RUP in primiparous cows. Reducing from 10 to 8% RDP decreased milk yield (10%) at 8% RUP, but increased yield (14%) at 6% RUP. Treatments did not affect yield of energy-corrected milk. For multiparous cows, treatments did not affect DMI. Reducing from 10 to 8% RDP decreased yield of energy-corrected milk (3.4%) at 8% RUP, but increased yield (8.8%) at 6% RUP. Reducing from 10 to 8% RDP and 8 to 6% RUP both increased N-use efficiency for primiparous and multiparous cows. The NRC model underestimated metabolizable protein and RUP supply, and overestimated RUP requirements, resulting in predictive losses of milk yield 1.4 to 5.8 times greater than observed values. In summary, the reduction of RDP and RUP proportions did not affect DMI, whereas the RUP reduction at 10% RDP had a small negative effect on energy-corrected milk yield. However, reduction of RDP and RUP consistently improved N-use efficiency of heat-stressed multiparous cows. The reduction of RDP and RUP proportions reduced DMI and milk yield but did not affect energy-corrected milk yield in primiparous cows, indicating a limited supply of nutrients.
本研究旨在探讨在炎热气候下，减少瘤胃可降解蛋白（RDP）和瘤胃非降解蛋白（RUP）比例对初产和经产奶牛采食量、产奶量和氮利用效率的影响。18 头初产和 30 头经产泌乳中期荷斯坦奶牛采用完全随机设计，处理采用 2×2 因子排列。奶牛随机分配到 4 种日粮处理中的 1 种，日粮设计为干物质（DM）中含有 2 种 RDP（10%和 8%）和 2 种 RUP（8%和 6%）比例，具体如下：（1）10% RDP，8% RUP；（2）8% RDP，8% RUP；（3）10% RDP，6% RUP；和（4）8% RDP，6% RUP。通过操纵蛋白质来源来获得所需的 RDP 和 RUP 比例。日粮等能，含 50%饲草和 50%精料（DM 基础）。奶牛在接受处理分配前 3 周单独饲喂 10% RDP、8% RUP 日粮。奶牛在田纳西州 7 月和 8 月的炎热天气下暴露于自由站立牛舍，没有补充冷却。使用 SAS（最小二乘均数±均值标准误；SAS Institute Inc.，Cary，NC）的混合程序测试主效应及其交互作用。观察到的养分摄入量和产奶量用于获得 NRC（2001）模型预测。研究过程中奶牛出现了热应激迹象。在 8% RUP 时，从 10%减少到 8% RDP 降低了干物质采食量（DMI；0.9kg/d），但在 6% RUP 时增加了 DMI（2.6kg/d）。在 8% RUP 时，从 10%减少到 8% RDP 降低了产奶量（10%），但在 6% RUP 时增加了产奶量（14%）。处理方式不影响能量校正奶的产量。对于经产奶牛，处理方式不影响 DMI。在 8% RUP 时，从 10%减少到 8% RDP 降低了能量校正奶的产量（3.4%），但在 6% RUP 时增加了产量（8.8%）。从 10%减少到 8% RDP 和 8%减少到 6% RUP 都提高了初产和经产奶牛的氮利用效率。NRC 模型低估了可代谢蛋白和 RUP 供应，高估了 RUP 需求，导致预测的产奶量损失比观察值大 1.4 至 5.8 倍。总之，降低 RDP 和 RUP 比例不影响 DMI，而在 10% RDP 时降低 RUP 对能量校正奶产量有轻微的负面影响。然而，降低 RDP 和 RUP 比例一致地提高了热应激经产奶牛的氮利用效率。降低 RDP 和 RUP 比例降低了 DMI 和产奶量，但对初产奶牛的能量校正奶产量没有影响，表明营养物质供应有限。
