The determination of leaf area index is a variable of great importance for predicting productivity, especially when it comes to forage species where the desired product is the leaves. This study aimed to determine the variation of the leaf area index of forage sorghum when subjected to different irrigation depths in four culture cuts. Treatments consisted of six irrigation depths, these being 0, 60, 80, 100, 120 and 140% of the reference evapotranspiration. Seven evaluations of leaf area index (LAI) were performed, which were in the four cuts, specifically at 50, 80, 110 and 140 days after sowing (DAS), and also 15 days after each cut to evaluate the culture’s resprouting, that is, at 65, 95 and 125 DAS. The delineation was randomized blocks, with six treatments and four blocks, totaling 24 experimental units. The experiment was conducted in Santiago-RS. Sowing was done on November 18, 2014. Using an irrigation system sprinkler, with one main row and six lateral rows, of PVC, being the irrigation levels differentiated by the nozzle diameter difference of sprinkler each lateral row. Samples of 0.5 linear meters per plot were collected in each of the seven evaluations, totaling 24 samples for evaluation. The analysis of leaf area index was performed using the computer program ImageJ. Significant results were found for the influence of the different irrigation levels on the leaf area index in the four cuts (recommended periods for grazing) evaluated, as well as for resprouting evaluations in the intermediate periods between cuts. Quadratic equations were adjusted for all the evaluation dates, where the 100 and 120% reference evapotranspiration levels alternated the largest LAI for all evaluations. The results also characterize that the second cut, or second grazing period of the irrigated forage sorghum, was the most productive, being it possible to recommend an increase in the animal load, or increase in grazing period in the area in this period. In this way, irrigation by sprinkling in the sorghum culture was characterized as an alternative for producers to increase leaf area index, and, consequently, crop productivity.
Key words: Sorghum bicolor L. (Moench), irrigation management, pasture irrigation, sprinkler irrigation, production estimation.
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