Context Beef industry productivity and profitability would be enhanced by improved efficiency at pasture. Our research is evaluating performance at pasture of Angus heifers divergent in estimated breeding values for residual feed intake determined from feedlot data (RFI-f-EBV) under a range of grazing conditions. Aims To determine whether Low- and High-RFI-f-EBV cattle differ in their growth response when pasture quality and availability become limiting to performance. Methods Eight-month-old heifers (n = 40) weaned at 6 months of age grazed within two replicates of 20, each with 10 low feedlot-efficiency (High-RFI-f-EBV) and 10 high feedlot-efficiency (Low-RFI-f-EBV) heifers. Each replicate grazed each of eight 1.25-ha paddocks comprising severely drought-affected, low-quality (mean dry-matter (DM) digestibility 44.1%, crude protein 7.3% DM, and 6.1 MJ metabolisable energy/kg DM) mixed perennial and annual native temperate grasses at 7-day intervals during repeated 28-day cycles, with Phase 1 with 2834 kg DM/ha and Phase 2 with 1890 kg DM/ha mean starting biomass. Heifers were yard-weighed weekly on nine occasions during the 8-week study. Key results During Phase 1 of grazing, the heifers gained 6.2 kg liveweight (LW) and during Phase 2 of grazing they lost 10 kg LW on average. Differences in LW between the RFI-f-EBV groups were not evident at the start or end of the study. However, over the 56 days of study, average daily change in LW calculated from the difference between starting and final LW was higher for Low-RFI-f-EBV heifers than for High-RFI-f-EBV heifers (–33 vs –127 g/day, s.e.m. = 41 g/day, P = 0.026). A similar result was evident when average daily LW change was determined from regression of LW on the day of study (–6 vs –96 g/day, s.e.m. = 41 g/day, P = 0.033). No significant interactions between grazing Phase and RFI-EBV group were evident for the growth responses. Conclusions Higher feedlot-efficiency (Low-RFI-f-EBV) weaner heifers maintained LW somewhat better than lower feedlot-efficiency (High-RFI-f-EBV) heifers, as the nutritional availability at pasture became more limiting. Implications Low-RFI-f-EBV weaner heifers may be more nutritionally resilient than are High-RFI-f-EBV heifers under drought conditions and, hence, may require less supplementary feed to maintain growth performance.

中文翻译：

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安格斯断奶小母牛的性能在剩余饲料摄入量-饲养场估计的育种值中变化严重干旱影响的牧场放牧

背景 提高牧场效率将提高牛肉行业的生产力和盈利能力。我们的研究正在评估安格斯小母牛在牧场上的表现，在一系列放牧条件下，根据饲养场数据 (RFI-f-EBV) 确定的剩余饲料摄入量的估计育种值存在差异。目的 确定当牧场质量和可用性成为性能限制时，低和高 RFI-f-EBV 牛的生长反应是否不同。方法 8 个月大的小母牛（n = 40）在 6 个月大时断奶，在 20 个重复中放牧，每个重复 10 个低饲养场效率（High-RFI-f-EBV）和 10 个高饲养场效率（Low- RFI-f-EBV) 小母牛。每个重复放牧 8 个 1.25 公顷的围场，这些围场包括受严重干旱影响的低质量（平均干物质 (DM) 消化率 44.1%，粗蛋白 7.3% DM，和 6.1 MJ 代谢能/kg DM）在重复的 28 天周期中以 7 天为间隔混合多年生和一年生天然温带草，第 1 阶段为 2834 kg DM/ha，第 2 阶段为 1890 kg DM/ha 平均起始生物量。在为期 8 周的研究中，每周有 9 次对小母牛进行院子称重。主要结果 在第 1 阶段放牧期间，小母牛增加了 6.2 公斤活重 (LW)，在第 2 阶段放牧期间，它们平均损失了 10 公斤活重 (LW)。在研究开始或结束时，RFI-f-EBV 组之间的 LW 差异并不明显。然而，在 56 天的研究中，根据起始和最终 LW 之间的差异计算出的 LW 平均每日变化对于低 RFI-f-EBV 小母牛高于高 RFI-f-EBV 小母牛（–33 vs –127 g/天，sem = 41 g/天，P = 0.026）。当从研究当天的 LW 回归确定平均每日 LW 变化时，类似的结果很明显（–6 对 –96 g/天，sem = 41 g/天，P = 0.033）。对于生长反应，放牧期和 RFI-EBV 组之间没有明显的相互作用。结论 较高的饲养场效率（Low-RFI-f-EBV）断奶小母牛比较低的饲养场效率（High-RFI-f-EBV）小母牛保持LW更好，因为牧场的营养供应变得更加有限。影响 在干旱条件下，低 RFI-f-EBV 断奶小母牛可能比高 RFI-f-EBV 小母牛在营养上更有弹性，因此可能需要较少的补充饲料来维持生长性能。对于生长反应，放牧期和 RFI-EBV 组之间没有明显的相互作用。结论 较高的饲养场效率（Low-RFI-f-EBV）断奶小母牛比较低的饲养场效率（High-RFI-f-EBV）小母牛保持LW更好，因为牧场的营养供应变得更加有限。含义 在干旱条件下，低 RFI-f-EBV 断奶小母牛可能比高 RFI-f-EBV 小母牛更具营养弹性，因此可能需要较少的补充饲料来维持生长性能。对于生长反应，放牧期和 RFI-EBV 组之间没有明显的相互作用。结论 较高的饲养场效率（Low-RFI-f-EBV）断奶小母牛比较低的饲养场效率（High-RFI-f-EBV）小母牛保持LW更好，因为牧场的营养供应变得更加有限。含义 在干旱条件下，低 RFI-f-EBV 断奶小母牛可能比高 RFI-f-EBV 小母牛更具营养弹性，因此可能需要较少的补充饲料来维持生长性能。