Forage response to simulated sheep grazing of leafy spurge
Hileman, Benjamin John
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Sheep grazing can lower leafy spurge biomass production; however, forage available for other domestic livestock or wildlife after sheep grazing is unknown. Our objective was to determine how forage biomass was affected by different sheep grazing patterns to answer the following questions: 1) will forage consumed by sheep while grazing leafy spurge be mitigated by higher forage biomass production resulting from lowering leafy spurge biomass production, 2) what timing and intensity of grazing maximizes forage biomass production and minimizes leafy spurge biomass production, 3) will defoliating leafy spurge result in higher forage biomass production by the end of the first grazing season, and 4) does the density of leafy spurge affect the response of forage biomass production. Three leafy spurge-infested sites in southeastern Montana were mechanically defoliated each summer for two or three years in a manner which mimicked sheep grazing. Plots were defoliated once or twice per growing season at pre-flowering, flowering, seed production, or pre-flowering and seed producing phenological stages of leafy spurge. Grasses, forbs, and leafy spurge were defoliated at two different intensities in irrigated and non-irrigated plots.High intensity defoliation of only forage species at flowering tended to result in lower forage biomass and higher leafy spurge biomass. On average, three years of high intensity defoliation of forage species at pre-flowering resulted in forage biomass sufficient to replace forage removed by sheep; three years of low intensity defoliation of forage species at pre-flowering and seed production resulted in forage biomass higher than baseline at all sites, but forage biomass production was not sufficient to replace all forage removed by sheep. These same treatments lowered the amount of leafy spurge biomass produced. High intensity defoliation of forage species and leafy spurge at pre-flowering, and at pre-flowering and seed production resulted in higher forage biomass at the end of the first grazing season at only one site. Leafy spurge density did not affect treatments. Therefore, sheep grazing at the proper timing and intensity can lower leafy spurge biomass and result in higher forage biomass production. However, care must be taken to avoid overgrazing desirable species after pre-flowering.