Intense winter grazing impairs subsequent Italian ryegrass re‐establishment in soybean‐pasture rotations

We evaluated Italian ryegrass (Lolium multiflorum Lam.) establishment in a factorial of grazing intensity (intense, moderate, moderate-light, light, and no grazing, with target average sward heights of 10, 20, 30, and 40 cm in the grazed plots) and additional seeding (self-seeding with or without additional seeding) in an integrated soybean-beef cattle system in southern Brazil. Grazing treatments were imposed in winter (June-July) and followed by a direct-seeded soybean crop (November). The establishment was quantified prior to the stocking periods in 2017 and 2018, corresponding to self-seeding from 2016 and 2017. Intense grazing impaired ryegrass re-establishment, resulting in larger individual plants (0.54 vs. 0.15 g dry matter plant(-1)) but lower population density (57 vs. 1,355 plants m(-2)) than in the other treatments. Additional broadcast ryegrass seeding did not fully compensate for the reduced natural reseeding in this treatment. The addition of seeds under intense grazing increased plant population density to values comparable to moderate grazing intensities but reduced individual plant mass, limiting herbage mass attained by the end of the pasture establishment phase. Contrastingly, the various combinations of individual plant mass and population density were sufficient to maintain herbage masses in moderate to light grazing intensities that were comparable to that in the ungrazed treatment, regardless of seed addition (63 vs. 180 g DM m(-2) for intense grazing vs. the average of other treatments over the whole establishment phase). Moderate grazing, e.g., target sward heights of 20 cm or greater, is necessary to achieve a system that can sustain itself without seed addition.