An evaluation of the performance and efficiency of nitrogen utilization in cattle fed tropical grass pastures with supplementation

The objective of this study was to evaluate the efficiency of nitrogen utilisation (ENU) and animal performance in cattle fed tropical grass pastures and receiving nitrogen supplementation, using a meta-analytical approach. The dataset used to evaluate the ENU was compiled from 10 experiments carried out in Brazil, which were published between 2009 and 2012, totalling 47 treatment means. To compose the dataset for animal performance evaluation, mean treatment values were collected from 44 experiments involving grazing beef cattle, which were published in Brazil from 2001 to 2012, totalling 182 treatment means. The relationships between variables were evaluated using linear or non-linear mixed models, taking into account the random variations among experiments. There was no association between ENU and the balance of nitrogen in the rumen and the efficiency of microbial synthesis in the rumen (P>0.05). The ENU was linearly associated with the dietary content of digestible organic matter in the diet (DOM) and relative production of microbial nitrogen in the rumen (NMICR), which was positive with DOM (P<0.04) and negative with NMICR (P<0.03). The relationships between ENU and dietary crude protein (CP) and the concentration of rumen ammonia nitrogen (RAN) were described using hyperbolic models (P<0.05). The estimated values that correspond to the apparent equilibrium point (i.e., ENU=0) were 108 g CP/kg of dry matter and 6.30 mg RAN/dL of rumen fluid. There was a positive response on weight gain as the amount of supplement increased (P<0.01). However, this relationship was interactively affected by CP content in the supplement (P<0.02) and in the forage (P<0.01). Through this way, the response of weight gain to supplement is improved by increasing the CP content of the supplement, but decreased as the CP in the forage increases. From this it can be conclude that there is a positive response to nitrogen supplementation with regards to ENU and animal performance. The main benefit of supplementation is the improvement of the nitrogen status in the animal׳s metabolism.