Irrigation threshold selection based on water productivity in a grazed Medicago sativa pasture

In Uruguay, there are 2.6 million hectares of improved pastures, of which only 2.4% are under irrigation. Drought periods are becoming increasingly frequent, significantly reducing forage production an issue that is further aggravated by the low water-holding capacity of local soils. Alfalfa is expected to respond positively to irrigation, achieving high production potential compared to many other forage species; however, results have been inconsistent. There is uncertainty regarding the optimal timing of irrigation to make the best use of rainfall, either by preventing evapotranspiration limitations or allowing them within certain limits to maximize productivity. The objective of this study was to determine the most suitable irrigation management strategy for grazed alfalfa, in terms of both the amount and timing of water applied. The trial was conducted at the Southern Regional Center, Faculty of Agronomy (Progreso, Canelones), where alfalfa cv. Chaná was sown at a rate of 20 kg·ha⁻¹. The experiment included three treatments and four replications. The treatments were: rainfed (no irrigation, T0); irrigation when 30% of available water was depleted (T30); and irrigation when 65% of available water was depleted (T65). Irrigation timing was determined by a water balance approach, supported by soil moisture measurements using an FDR probe to verify irrigation thresholds. Grazing timing was based on the 8-node criterion, with a uniformity cut at 5 cm height after grazing. The significance level used was 0.05. Annual cumulative yield showed no significant differences among treatments, with an average of 23,500 ± 979 kg DM·ha⁻¹. Differences between treatments were detected during the second summer in favor of the irrigated treatments, despite it being classified as a summer with low atmospheric demand. Regarding water productivity, T0 and T65 showed significant differences compared to T30 (52–41 vs. 37 kg DM·mm⁻¹). The maximum threshold for the relationship between biomass produced and water consumed was determined to be 500 mm·year⁻¹.