UNGULATE GRAZING IN SAGEBRUSH GRASSLAND - EFFECTS OF RESOURCE COMPETITION ON SECONDARY PRODUCTION

In many areas of western North America, populations of elk are believe d to harm production of cattle by competing with them for limited supp lies of native forage. We examined effects of variation in the populat ion density of elk (Cen,ns elaphus canadensis) during winter on growth and reproduction of cattle during spring, using a randomized complete block experiment conducted in sagebrush grassland during four years. We manipulated elk numbers to achieve four levels of population densit y (0, 9, 15, and 31 elk/km(2)), replicated each level three times, and observed responses of cattle to these manipulations. Mean birth dates of calves born to cows in the intermediate (9, 15 elk/km(2)) treatmen t levels were delayed by 5 d relative to birth dates of calves born to cows in controls (0 elk/km(2)), but these trends were not statistical ly significant (quadratic effect F-1,F-6 = 3.2, P = 0.13). Calf body m ass at birth was not significantly influenced by treatment, but calf b ody mass at the end of spring declined linearly (F-1,F-6 = 7.9, P = 0. 03) with increasing elk population density, from a mean of 80.2 kg in the control to 73.0 kg in the 31 elk/km(2) treatment. Calf body mass a t weaning was weakly depressed by treatment, with the largest treatmen t effects occurring at the 9 elk/km(2) level (F-1,F-6 = 8.8, P = 0.02) . Body mass of cows at the end of the spring grazing season tended to decline linearly with treatment, but these tendencies were not statist ically significant (linear contrast F-1,F-6 = 1.9, P = 0.22). We did n ot find significant effects of treatment on cow body mass at time of w eaning (control vs, others contrast F-1,F-6 = 3.3, P = 0.12) or on nat ality rates (control vs. other contrast F-1,F-6 = 1.2, P = 0.31), alth ough, in both cases, values for the control tended to exceed the treat ments. Body mass of cows (t(1) = -3.9, P = 0.0003) and calves (t(1) = -3.9, P < 0.0001) at the end of the spring grazing season were quadrat ically related to the biomass of available herbaceous forage during sp ring. We observed a threshold in effects of forage supply on cattle pr oduction at about 45 g/m(2) of live and dead herbaceous biomass. Cattl e production declined with declining forage biomass when forage supply fell below this threshold. Total cattle production (kilograms per cow per year), was quadratically related to elk population density (F-1,F -6 = 5.8, P = 0.05), Average cattle production in the control ((X) ove r bar = 248 kg . cow(-1). yr(-1)) exceeded the mean of the other treat ment levels ((X) over bar = 224 kg . cow(-1). yr(-)1, F-1,F-6 = 6.7, P = 0.04). Quadratic responses in cattle performance were apparently ca used by compensatory growth after the spring grazing season: growth ra tes of cows during spring were inversely related to their subsequent g rowth rates during summer and fall (F,,, = 33.5, P < 0.0001, r(2) = 0. 50). We conclude that elk grazing caused reductions in cattle producti on, but the magnitude of the effects were not proportionate to elk pop ulation density. Our studies revealed that competition between cattle and elk operates in a threshold fashion. If sufficient forage is avail able to cattle following elk grazing, elk populations will not harm ca ttle performance, In the system we studied, this threshold occurred at approximate to 45 g/m(2) of live and dead herbaceous biomass.