Free-ranging domestic livestock compete with wild grazing mammals for forage. In this study, an attempt was made to ascertain best sheep-grazing practice on sagebrush grass habitat to reduce competition with ‘big game‘, which use the area in winter. Grazing trials were undertaken on the Hardware Ranch near Logan city, Utah (mid-west USA). Rangeland was sheep-grazed at different times to ascertain what seasons and intensity of use would maximize grazing of herbs whilst minimising browsing of shrubs which provide most big game forage in winter.
From 1967 through 1969, 12 pastures (each approximately 3 acres; 1.2 ha) were arranged in pairs and each pair was grazed over one of six 8-16 day periods. Beginning dates for grazing were around 20 May, 20 June, 15 July, 10 August, 5 September and 1 October each year. Grazing intensities were alternated each year on the pasture pairs.
In 1967, grazing intensities were 40% (moderate) and 70% (heavy) utilization of the two assumed key herb forage species, Pacific aster Aster chilensis and arrowleaf balsamroot Balsamorhiza sagittata. Grazing intensities assigned in 1968 and 1969 were based on pre-grazing estimates of ‘usable’ forage.
Bitterbrush Purshia tridentata was the most desirable and abundant shrub for big game. Sheep browsing intensity was recorded by measuring twigs on four branches on each of 50 plants per pasture. In 1968 and 1969 in adjacent areas, 30 bitterbrush plants were selected to determine rate of twig growth when sheep were excluded.
From late spring to early summer sheep ate mostly herbs, and the fairly light browsing of shrubs (e.g. average utilization over the 3 study years for bitterbrush: moderate intensity 22-23 %; heavy 32-45%) observed resulted in little or no reduction in shrub forage production at the end of the growing season (autumn).
After mid-July, sheep heavily browsed bitterbrush (average utilization (over 3 years): moderate intensity 33-72%; heavy 60-87%) and it did not recover fully before winter set in, thus over-winter bitterbrush forage available for big game was reduced.
Results indicate that sheep grazing prior to mid-July at the intensities trialed did not adversely reduce the quantity of bitterbrush and other shrub over-winter forage available to big game. There was no evidence that subsequent annual productivity of shrubs and plants was impaired by any of the grazing regimes.
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