Grazing Ecology Research Group, Federal University of Rio Grande do Sul, Porto Alegre, Rio Grande do Sul 91540-000, Brazil.
J Anim Sci. 2013 Sep;91(9):4357-65. doi: 10.2527/jas.2012-5602. Epub 2013 Jul 3.
To maximize herbage DMI, pregrazing sward surface height (SSH) and level of herbage depletion (HD) must be such that variables determining short-term herbage intake such as bite mass (BM) and bite rate (BR) are optimized. The objective of this study was to determine a SSH target and the level of HD as a proportion of the SSH that optimizes BM and BR of beef heifers grazing Sorghum bicolor swards. Two experiments were conducted using 2 S. bicolor swards and 4 beef heifers (25 mo old; 322 kg BW). Experiment 1 compared the effect of 6 pregrazing SSH, 30, 40, 50, 60, 70, and 80 cm, on BM, BR, and jaw movements. Experiment 2 assessed the effect of HD level as a proportion of SSH (0.17, 0.34, 0.50, 0.67 and 0.84) on BM, BR, and jaw movements using the optimal pregrazing SSH defined in Exp. 1. Short-term herbage DMI was estimated using a double-weighing technique and corrected for insensible BW loss. Herbage DMI was subsequently used to calculate the BM. Net eating time and jaw movements for apprehension and manipulation + mastication during grazing as well as total jaw movements were determined using the IGER (Institute of Grassland and Environmental Research) behavior recorders. Bite rate and the number of total jaw movements per gram herbage DMI were derived from jaw movement count and measurements of herbage DMI. The results of Exp. 1 showed low and high SSH constraint the ease of herbage harvesting. Greater BM are maintained until a SSH of 50 cm is reached (P < 0.05) and then decline at greater SSH due to herbage dispersion. The nonbiting jaw movement rate increased at greater SSH whereas BR decreased (P < 0.05). For both variables, the turning point was close to a SSH of 50 cm. Experiment 2 showed that such an optimization of BM and BR was maintained until an HD level of 0.34 was reached (P < 0.05). There was a linear increase in both the total jaw movements per unit herbage DMI and the nonbiting jaw movements rate (manipulation + mastication) subsequent to levels of HD greater than 0.34 (P < 0.05). These studies provide, for the first time, sward feature targets to manage grazing and optimize BM and BR, aiming to maximize the short-term herbage DMI of cattle grazing S. bicolor swards.
为了使采食最大化，放牧前草地表面高度（SSH）和牧草耗竭水平（HD）必须优化，以确定决定短期牧草采食量的变量，如咬质量（BM）和咬率（BR）。本研究的目的是确定一个 SSH 目标和 HD 水平，该水平占 SSH 的比例可优化采食 Sorghum bicolor 草地的肉牛的 BM 和 BR。使用 2 个 S. bicolor 草地和 4 头肉牛（25 月龄；322 kg BW）进行了 2 个实验。实验 1 比较了 6 个放牧前 SSH（30、40、50、60、70 和 80 cm）对 BM、BR 和咀嚼运动的影响。实验 2 使用实验 1 中定义的最优放牧前 SSH，评估了 HD 水平（SSH 的 0.17、0.34、0.50、0.67 和 0.84）对 BM、BR 和咀嚼运动的影响。使用双重称重技术估计短期牧草干物质采食量，并校正无感觉 BW 损失。随后，根据牧草干物质采食量计算 BM。使用 IGER（草地与环境研究所）行为记录器测定放牧时的净采食时间和用于采食、试探和咀嚼的咀嚼运动以及总咀嚼运动。从咀嚼运动计数和牧草干物质采食量测量中得出咬率和每克牧草干物质采食量的总咀嚼运动次数。实验 1 的结果表明，低和高 SSH 限制了牧草收获的难易程度。在达到 50 cm 的 SSH 之前，保持较高的 BM（P < 0.05），然后在更高的 SSH 下下降，因为牧草分散。在更大的 SSH 时，非咀嚼的咀嚼运动速度增加，而 BR 下降（P < 0.05）。对于这两个变量，转折点接近 50 cm 的 SSH。实验 2 表明，在达到 HD 水平 0.34 之前，BM 和 BR 保持这种优化（P < 0.05）。在 HD 水平大于 0.34 后，总咀嚼运动次数/单位牧草干物质采食量和非咀嚼咀嚼运动率（采食+咀嚼）呈线性增加（P < 0.05）。这些研究首次提供了管理放牧和优化 BM 和 BR 的草地特征目标，旨在使采食 S. bicolor 草地的牛的短期牧草干物质采食量最大化。
