Annual winter pastures are strategic components in crop-livestock systems in Uruguay. The aim of this study was to evaluate the N legume fixation into an annual ryegrass pasture contrasted to the N fertilizer addition. The experimental design was a randomized complete block design with 2×2 factorial arrangement of the treatments, two pasture types: a) annual ryegrass (Lolium multiflorum L.) pasture, and b) of annual ryegrass with two annual legume species (Trifolium resupinatum L. and Trifolium vesiculosum L.); and two levels of N fertilizer: a) no N fertilizer, and b) 64 kg N ha-1. They were grazed with Holstein steers. Setaria italica crop was planted in summer to evaluate the carry over effects of the treatments on its herbage mass production. Pasture accumulated herbage mass was significantly increased with N fertilizer (P ≤ 0.01), increment of 1277 ± 322.5 kg DM ha-1. The legume species’ addition plus N fertilization led to a higher nitrogen nutrition index in the mixed pastures. Ryegrass herbage mass was positively influenced by N addition, but with that was a 47% of weed herbage mass increase in the pure annual ryegrass pastures. The average daily gain of steers was positively influenced by legumes, with an average of 1.01 kg PV day-1 in mixtures. N fertilizer addition allowed a higher stocking rate and higher animal production in winter. A carryover effect of the treatments was not found on S. italica accumulated herbage mass. Legumes’ addition to annual ryegrass allowed a greater animal performance but did not provide any additional benefit to the following summer crop.

Keywords: nitrogen use, BNF, legumes incorporation, NNI (Nitrogen Nutrition Index).

Contact: García-Favre, J. javigarciafavre@gmail.com