Department of Animal Science, Michigan State University, East Lansing 48824.
Department of Animal Science, Michigan State University, East Lansing 48824.
J Dairy Sci. 2020 Dec;103(12):11401-11412. doi: 10.3168/jds.2020-18143. Epub 2020 Oct 23.
Our objectives were to determine (1) the sources of variation in cow responses to dietary protein reduction, and (2) the association of low dietary protein resilience (LPR) with protein efficiency. Lactating Holstein cows in peak lactation (n = 166; 92 primiparous, 77 multiparous) with initial milk yield 41 ± 9.8 kg/d were fed high-protein (HP) or low-protein (LP) diets in 4-wk periods in a crossover design with half the cows fed LP first and half fed HP first. The study was repeated for 69 of these cows (42 primiparous, 27 multiparous) in late lactation. Low-protein diets were 14% crude protein (CP) in peak lactation and 13% CP in late lactation and were formulated to contain adequate rumen-degradable protein to maintain rumen function but inadequate rumen-undegradable protein for cows with average production in this study to maintain their production. High-protein diets were 18% CP in peak lactation and 16% CP in late lactation and contained extra expeller soybean meal to meet metabolizable protein requirements. Protein efficiency was defined as the protein captured in milk or in both milk and body tissues per unit of consumed protein. Low dietary protein resilience was calculated for each cow in peak and late lactation based on actual intake, production, and body weight measures. The ability of a cow to maintain total protein captured in milk and body gain when fed less protein varied considerably and the variation was mostly explained by a cow's captured protein per kilogram of metabolic body weight when fed HP, her parity, treatment sequence, and experiment. Protein efficiency was moderately repeatable across diets within lactation stage. Milk urea nitrogen was not associated with protein efficiency in individual cows within a diet and lactation stage. Cows with greater dietary protein resiliency (higher LPR) had similar protein efficiency on the HP diet as cows with lower LPR, but higher protein efficiency on the LP diet. In conclusion, cows generally maintained their protein efficiency rankings when switching diets between sufficient or insufficient protein; however, some high-producing cows are better able to maintain high production when fed less protein. We define this ability as LPR and suggest it might be useful for identifying cows that use protein more efficiently to enhance dairy sustainability.
我们的目标是确定（1）奶牛对日粮蛋白减少的反应变化的来源，以及（2）低日粮蛋白恢复力（LPR）与蛋白效率的关系。处于泌乳高峰期的荷斯坦奶牛（n = 166；92 头初产，77 头经产），初始产奶量为 41 ± 9.8kg/d，采用高蛋白质（HP）或低蛋白质（LP）日粮进行 4 周交叉设计，其中一半奶牛先饲喂 LP，另一半先饲喂 HP。这项研究在 69 头处于泌乳后期的奶牛中重复进行（42 头初产，27 头经产）。泌乳高峰期 LP 日粮的粗蛋白（CP）含量为 14%，泌乳后期为 13%，并配制为含有足够的瘤胃可降解蛋白以维持瘤胃功能，但不足以维持本研究中平均产奶量的奶牛的产奶量。HP 日粮在泌乳高峰期的 CP 含量为 18%，在泌乳后期为 16%，并添加了额外的膨化大豆以满足可代谢蛋白的需求。蛋白效率定义为每单位摄入、生产和体重所消耗的蛋白质中捕捉到的蛋白质。根据实际摄入、生产和体重测量值，计算了泌乳高峰期和泌乳后期每头奶牛的低日粮蛋白恢复力。当饲喂较少的蛋白质时，奶牛维持牛奶和体组织中总蛋白的能力差异很大，这种差异主要由奶牛在饲喂 HP 时每公斤代谢体重所摄入的蛋白质、其胎次、处理顺序和实验来解释。在泌乳阶段内，蛋白效率在不同日粮之间具有中等的可重复性。在单个奶牛的单个日粮和泌乳阶段内，牛奶尿素氮与蛋白效率无关。具有较高日粮蛋白恢复力（较高的 LPR）的奶牛在 HP 日粮上的蛋白效率与 LPR 较低的奶牛相似，但在 LP 日粮上的蛋白效率更高。总之，当奶牛在充足或不足的蛋白质日粮之间切换时，它们通常能保持其蛋白效率的排名；然而，一些高生产奶牛在饲喂较少的蛋白质时能够更好地维持高生产。我们将这种能力定义为 LPR，并建议它可能有助于识别更有效地利用蛋白质以提高奶牛可持续性的奶牛。
