The use of legumes under innovative grazing methods and strategic nutrients supplementation for cattle in tropical grazing systems

牧草是世界各地反刍动物的主要饲料来源。然而，放牧动物的生产力通常受到低氮(N)供应的限制，这阻碍了瘤胃内微生物蛋白质的合成。放牧制度下的氮素供应可以通过在草地上种植豆科植物来增加，也可以通过向动物补充浓缩氮源来增加，或者通过两种策略的结合来增加。豆科植物已经成功地融入温带放牧系统，但它们融入热带系统却不太成功。这与豆科植物容易被践踏有关，也与这些物种优先放牧牲畜有关，这增加了豆科植物的压力。此外，牧场管理策略是在植物垂直落叶的基础上从专属牧场采用的，可能会导致豆类植物极度落叶，进一步限制将豆类植物纳入热带放牧系统。在豆科牧草被纳入放牧系统的情况下，动物的营养状况得到了明显的改善，但由于动物的优先放牧，观察到了N的浪费，这与高效利用自然资源的目的背道而驰。因此，本研究的目的是：1)评价一种基于豆科牧草混交场牧草水平利用的新型轮牧管理；2)研究N补充与牧草之间的相互作用对放牧牛消化动力学的影响。因此，本研究的目的是：1)评价基于水平利用豆科牧草群牧草的新型轮牧管理模式；2)研究施氮量与牧草对放牧牛消化动力学的影响在第一项研究中，泽布公牛将在试验田上只吃草或混合豆科牧草群。将基于未放牧草地水平比例的轮作方案与基于牧草高度的控制轮作方案进行比较。将使用正构烷烃标记法估计总摄入量和每种物质的消耗量比例。第二个试验用泽布公牛在单一牧草或豆科牧草草地上放牧，并添加有机或无机氮源，以评估固体和液体瘤胃消化的部分流出速率。这将通过为公牛安装一个蠕动泵来实现，在放牧期间，不断向公牛注入铬和Yb-传递率的标志物。微生物蛋白质合成和饮食选择将分别根据嘌呤衍生物排泄量和正构烷烃方法进行评估。这项研究将更好地了解豆科牧草混合牧场放牧过程中的动植物相互作用；也将为豆科牧草成功纳入热带放牧系统提供改进的牧场管理策略。此外，对牛在豆科牧草混合牧场上放牧时瘤胃动态的生理反应将产生重要的理解，这受补充N的特性的影响。

Grasses are a main feed source for ruminants across the world. However, the productivity of grazing animals is commonly limited by a low supply of nitrogen (N), which hinders microbial protein synthesis in the rumen. The N supply under grazing systems can either be increased by incorporating legume species into the sward, by supplementing animals with concentrated N sources, or by a combination of both strategies. Legumes have been successfully incorporated into temperate grazing systems, but their incorporation into tropical systems has been less successful. This is related to the legumes’ susceptibility to trampling as well as to the preferential grazing of livestock for these species, which adds pressure to legumes. Additionally, pasture management strategies, which are adopted from exclusive grass swards based on the vertical defoliation of plants, can cause extreme defoliation of legumes, further limiting the incorporation of legumes into tropical grazing systems. In cases when legumes have been incorporated into the grazing system, an improvement in the nutritional status of animals has become clear, but, because of the preferential grazing by animals evidence of N wastage has been observed, a development opposite to the aim of efficiently using natural resources.Therefore, the aims of this research are i) to evaluate an innovative rotational grazing management based on the horizontal utilization of pastures in mixed legume-grass swards, and ii) to the study the interactions between N supplementation and the forages available in the pasture on the digesta kinetics of grazing cattle.For this, in a first study Zebu bulls will graze on experimental plots of only grasses or mixed legume-grass swards. A rotational scheme based on the horizontal proportion of un-grazed pasture will be compared with a control rotation scheme based on pasture height. The n-alkanes marker method will be used to estimate total intake and the proportion of each species consumed. A second trial with Zebu bulls grazing in sole grass or grass- legume tree pastures and supplemented with organic or inorganic sources of N will evaluate the fractional outflow rate of solid and liquid rumen digesta. This will be done by fitting the bulls with a peristaltic pump to constantly infuse Chromium and Ytterbium –markers for the passage rate - during grazing. Microbial protein synthesis and diet selection will be estimated from the purine derivatives excretion, and the n-alkanes methodology, respectively. The research will produce a better understanding of the plant × animal interactions during grazing of mixed legume-grass swards; it will also provide an improved pasture management strategy for a successful incorporation of legumes into tropical grazing systems. Moreover, important understanding will be generated on the physiological responses of the rumen dynamics of cattle, when grazing on mixed legume-grass swards, as affected by the characteristics of the supplemental N.