Efecto de la intervención de campo natural con nitrógeno y leguminosas en verano otoño

This research was carried out in the 18^th paddock of the Mario Alcides Cassinoni Experimental Station, of the School of Agronomy, located on the 363^rd km of the National Route No. 3 “General Artigas”, in Paysandú department, Uruguay (32º 20’ 9” South latitude, and 58º West Longitude, 61 ms. N.m). The evaluation period was between October 12th. 2017 and April 18th. 2018, and was divided in 3 sub-periods: spring, summer and autumn. The objective of the experiment was to evaluate the response of the natural field under rotational grazing to the introduction of legumes and fertilization with two doses of nitrogen in forage production, botanical composition, and secondary production.A randomized complete block design was used with 4 repetitions, the treatments being: 1- Natural field without intervention (CN), 2- Natural field improved with Lotus tenuis cv Matrero and Trifolium pratense cv. Estanzuela (CNM), 3- natural field fertilized with 60 kg/ha of N per year (N60), and 4- natural field fertilized with 120 kg/ha of N per year (N120). The three treatments that were intervened were also fertilized with 40 units of P2O5 /ha per year. Fertilization history and nitrogen dose were also evaluated. The variables related to the pasture studied were dry matter produced (MSProd), growth rate (TC), dry matter available (MSD), remaining dry matter (MSR), dry matter disappeared (MSDes), and the variation of the botanical composition. Meanwhile, the evaluations of the animal component were carried out on the variables instantaneous load (CI), average load (CM) and average daily gain (GMD). The results show that nitrogen fertilization increased the MSProd and TC with respect to the CN, a difference that was not observed significant within any of the subperiods but was observed for the entire study period, a behavior that is largely explained by the water restrictions that were presented during the experiment and the high loads supported by nitrogenous treatments, especially during spring and summer. Meanwhile, the CNM presented intermediate values ​​between the control and the treatments fertilized with N, without significantly differentiating from any of them. Within the subperiods, neither did the MSD, MSR or MSDes vary between treatments, although a decrease was observed from each period to the next for the first two variables. Regarding botanical composition, an increase in the participation of winter annual grass species was observed to the detriment of summer perennial grass species in nitrogenous treatments with respect to CN, especially in N120. Meanwhile, legumes did not show a difference in their participation within CNM or any of the treatments without fertilization history, but instead a positive effect of fertilization history was observed on the participation of this family of species in nitrogen treatments, particularly during the summer. Within secondary production, nitrogen treatments supported loads 100% higher than CN and CNM during spring and summer, with N120 showing the poorest animal performance for these subperiods. For the fall, these differences in loads were corrected and said treatment became the one with the best animal performance, keeping the GMD of the other three treatments at values ​​of similar magnitude to each other for the entire period.