Effect of irrigation and grazing on the production of an alfalfa pasture (Medicago sativa L.)

Alfalfa is a forage species widely used in Uruguay in intensive livestock and dairy farming. Supplementary irrigation makes it possible to mitigate the negative effects of climate change, increasing and stabilizing the productivity of pastures. It is necessary to evaluate its joint effect with grazing. Two irrigation thresholds were evaluated, with entry of animals. The treatments were: frequent irrigation (30% depleted), spaced irrigation (65% depleted) and rainfed. The experimental design was complete randomized blocks, and 30 months of evaluation. The average annual yield of irrigated alfalfa was 25,333 and 23,177 kg DM ha-1, first and second year of evaluation, respectively. The accumulated production (30 months) did not show a significant difference between the irrigated treatments, but it did between irrigated and rainfed. In the last summer-autumn, the yield of all treatments fell to 8660 kg DM ha-1 (irrigated) and 1756 kg DM ha-1 (rainfed). Seasonal productivity indicates the convenience of irrigating alfalfa in summer since the yield increased 43% in the second year and 4.3 times more in the last year. Furthermore, after the extreme summer drought, in the following autumn, the irrigated treatments yielded 7 times more than the rainfed. Management with spaced irrigation and irrigation depth that do not return the soil to its field capacity increases the effectiveness of precipitation, saving water and energy allocated to irrigation. The apparent density increased towards the end of the experiment as a result of grazing, however, there was no difference between the evaluated irrigation managements.