Efecto de la fertilización nitrogenada e incorporación de leguminosas sobre la productividad y composición botánica de un campo natural

This work was carried out in pasture 18 of the Mario A. Cassinoni Experimental Station (EEMAC) at the Faculty of Agronomy, University of the Republic. It is located on National Route No. 3, General Brigadier José Gervasio Artigas, km 363, Paysandú Department, Uruguay (32º 20' 9'' South latitude and 58º West longitude, 61 m above sea level). The evaluation period was from June 8, 2015, to February 9, 2016, and it was divided into three periods: winter, from June 8, 2015, to October 1, 2015; spring, from October 1, 2015, to December 18, 2015; and summer, from December 18, 2015, to February 9, 2016. The objective of the study was to evaluate dry matter production and changes in the botanical composition of a natural pasture subjected to different levels of intervention, under grazing by steers with a forage offer ranging between 8% and 10% of live weight. The experimental design was a completely randomized block design with four treatments. These included a control without intervention (CN), a treatment improved by the incorporation of Lotus tenuis and Trifolium pratense with a fertilization of 40 kg per hectare of P2O5 (CNM), and two treatments with the same phosphorus fertilization of 40 kg P2O5 per hectare but different nitrogen doses: one with 60 kg nitrogen per hectare (60N) and the other with 120 kg nitrogen per hectare (120N). The studied variables included: dry matter production (PROD), available dry matter (MSDISP), percentage of available green dry matter (%MSDISPV), percentage of available dry matter remains (%MSDISPRS), daily growth rate (TC), disappeared dry matter (MSDESP), percentage of disappeared forage (%DESP), available height (ALTDISP), remaining dry matter (MSREM), remaining height (ALTREM), percentage of remaining green dry matter (%MSRSV), and percentage of remaining dry matter remains (%MSREMRS). The botanical composition analysis studied the following variables: percentage of green dry matter (%MSV), percentage of dry matter remains (%MSRS), percentage of winter perennial grasses (%GPI), percentage of summer perennial grasses (%GPE), percentage of winter annual grasses (%GAI), percentage of herbs (%HIERB), percentage of legumes (%LEG), and percentage of thistles (%CARD). The results indicate that nitrogen fertilization, both at 60 kg and 120 kg per hectare, significantly increased dry matter production in winter and spring, leading to a greater amount of disappeared forage. Additionally, the treatment improved with legumes increased dry matter production during the summer period compared to the non-intervention treatment. Regarding the botanical composition, a higher presence of legumes was observed in the improved natural pasture treatment (CNM). The study also showed the seasonal variation of the botanical components: in winter, winter perennial grasses, herbs, and thistles increased, while in spring, summer perennial grasses and legumes increased. Furthermore, spring showed a higher proportion of green forage compared to winter.