Department of Animal Science, Michigan State University, East Lansing 48824.
Department of Animal Science, Michigan State University, East Lansing 48824.
J Dairy Sci. 2020 Apr;103(4):3177-3190. doi: 10.3168/jds.2019-17567. Epub 2020 Feb 12.
Our objectives were to determine the repeatability of residual feed intake (RFI) across dietary protein levels and to determine the association between RFI and protein efficiency in lactating cows. Holstein cows (n = 166; 92 primiparous, 74 multiparous) with initial milk yield 41.3 ± 9.8 kg/d were fed diets with high or low protein in peak lactation. Experiments were conducted as crossovers with 2 treatment periods of 28 to 35 d. Production of 69 of the 166 cows (42 primiparous, 27 multiparous) was also measured 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. High-protein diets were 18% CP in peak lactation and 16% CP in late lactation and contained extra expeller soybean meal to increase absorbed protein. Cows were milked twice daily; DMI and milk yield were recorded daily. Milk composition was measured over 4 consecutive milkings weekly, and body weight (BW) was measured 3 times weekly. Fixed effects of diet, parity, and treatment period, interaction of parity and diet, and random effects of experiment and cow nested within experiment were included in the model to compare intake and production performance between cows fed different levels of CP. The RFI value was calculated for each cow on each treatment based on the actual intake, milk energy output, metabolic BW, and body energy (calculated from BW change and body condition score over the treatment period) change. Ranking of cows for RFI was moderately repeatable across dietary protein in peak lactation (r = 0.59) but less repeatable in late lactation (r = 0.41). Negative correlation was observed between RFI and protein efficiency values (dietary protein captured in milk) for cows in both peak lactation (r = -0.42) and late lactation (r = -0.24), which suggested that cows with higher energy efficiency had greater protein efficiency. In conclusion, RFI was repeatable across dietary protein levels within lactation stage, and cows with lower RFI values utilized protein more efficiently.
我们的目标是确定在不同日粮蛋白质水平下剩余采食量 (RFI) 的可重复性，并确定泌乳奶牛的 RFI 与蛋白质效率之间的关系。具有 41.3±9.8kg/d 初始产奶量的荷斯坦奶牛 (n=166;92 头初产，74 头经产) 在泌乳高峰期饲喂高蛋白或低蛋白日粮。试验以交叉设计进行，有 2 个 28-35d 的处理期。在泌乳后期，对 166 头奶牛中的 69 头 (42 头初产，27 头经产) 的生产性能进行了测量。泌乳高峰期低蛋白日粮粗蛋白 (CP) 含量为 14%，泌乳后期 CP 含量为 13%，并配制了足够的瘤胃降解蛋白以维持瘤胃功能。泌乳高峰期高蛋白日粮 CP 含量为 18%，泌乳后期 CP 含量为 16%，并添加了膨化大豆以增加可吸收蛋白。奶牛每天挤奶 2 次；每天记录干物质采食量和产奶量。每周连续 4 次测量牛奶成分，每周 3 次测量体重 (BW)。模型中包括日粮、胎次和处理期的固定效应、胎次和日粮的互作以及实验内嵌套的实验和奶牛的随机效应，以比较不同 CP 水平奶牛的采食量和生产性能。根据实际采食量、乳能输出、代谢 BW 和处理期内 BW 变化和体况评分计算的体能，为每头奶牛在每个处理上计算 RFI 值。泌乳高峰期日粮 CP 水平下，奶牛 RFI 值的排序具有中等可重复性 (r=0.59)，但泌乳后期的可重复性较低 (r=0.41)。泌乳高峰期 (r=-0.42) 和泌乳后期 (r=-0.24) 奶牛的 RFI 与蛋白质效率值 (乳中摄入的蛋白质) 呈负相关，这表明能量效率较高的奶牛蛋白质效率更高。总之，RFI 在泌乳阶段内的日粮蛋白质水平上具有可重复性，且 RFI 值较低的奶牛更有效地利用蛋白质。
