Productividad primavero estival de campo natural, campo natural mejorado con leguminosas y fósforo o fertilizado con nitrógeno y fósforo

The experiments were carried out at the Mario A. Cassinoni Experimental Station of the “Facultad de Agronomía – UdelaR” located on 363^rd km of the “General Artigas” route, Paysandú, Uruguay; precisely 10,3 ha being part of stockyard 18. The evaluation period was from september 17^th of 2020, to march 5^th of 2021. This period was divided into three sub-periods: Spring (17/9/20 – 14/11/20), Spring-Summer (14/11/20 – 10/1/21); and Summer (10/1/21 – 5/3/21). The objective was to evaluate the productive response from Sping to Summer of the natural field with different levels of intensification, the introduction of legumes with phosphorus fertilization or two dosage of nitrogen fertilization and phosphorus fertilization, under rotational grazing by a forage offered of 10-12%. The experiment design used for experiment 1 was completely randomized blocks with four repetitions. The treatments were: without intervention (CN), an improvement with Lotus tenuis and Trifolium pratense (CNM), and two levels of nitrogen fertilization, 60 kgN/ha/y (N60), and 120 kgN/ha/y (N120) applied in autumn-winter. Besides, except the control treatment (CN) all the others were fertilized with phosphorus by 40 kgP2O5/ha/y in autumn. Otherwise, the experimental design used for experiment 2 was also completely randomized blocks with four repetitions, save that in this time the treatments were two levels of nitrogen fertilization: 60 kgN/ha/y and 120 kgN/ha/y applied in autumn-winter; both under phosphate fertilization once a year by 40 kgP2O5/ha in autumn. In this experiment, the effect of the fertilization history was also evaluated. The analyzed variables were: present forage, forage production, daily growth rate, available forage, remaining forage, available and remaining height, disappeared forage, botanical composition, relation between green forage and dry forage of available and remaining forage, percentage of bare ground, ground covered by mulch, and ground covered by dirty field weeds, live weight, stocking rate, average daily gain, gain per hectare, and forage allowance. As a result of experiment 1, it was observed that the N120 treatment was able to increase the forage production in comparison with CN and CNM, without showing significant differences with 60N treatment. Regarding the secondary production, no statistically significant differences were detected between the treatments for the total period in the average daily gain of the animals. However, the intervention of the natural field, either with the addition of legumes or the nitrogenous fertilization, provided the animals with higher quality forage, because even though they had lower forage offer (OF) they obtained statistically non-different gains. Both the addition of legumes and nitrogen generated a change in the botanical composition. For the total period evaluated, Lolium multiflorum was significantly higher in the CNM, N60 and N120 treatments, on the other hand, Paspalum dilatatum presented a significantly higher contribution in the CNM treatment compared to N120, without denoting significant differences with the CN and N60 treatments. As a result of experiment 2, it was obtained that the longer fertilization history generated a lower spring-summer forage production, in response to the substitution of summer perennial grasses (Paspalum dilatatum and Paspalum notatum) by dwarf weeds and MCS.