NATIVE OR SEEDED RANGELAND FOR COWS WITH HIGH OR LOW MILK-PRODUCTION

Multiparous cows (n = 91, 1986; n = 92, 1987) were selected from 2 pop ulations to obtain cattle with high and low milk production. After Mar ch-April calving, high and low producing cows grazed either native ran ge (treatment 1) or seeded range (treatment 2) until weaning in Septem ber. Seeded range included paddocks of crested wheatgrass (Agropyron d esertorum, Fisch. ex [Link]Schult), contour furrowed native range inte rseeded with ''Ladak'' alfalfa (Medicago Sativa L.), and Russian wildr ye grass (Psathrostachys juncea [Fisch.] Nevski.). In treatment 2, cre sted wheatgrass was grazed mid-April to 17 June, followed by contour f urrowed rangeland 18 June to 5 August, and ended with Russian wildrye 6 August to weaning (mid-September). Data were analyzed as a split plo t with treatment and year in the main plot and cow type in the subplot . Number of cows exhibiting estrus before the beginning of the breedin g season and fall pregnancy rate were not influenced (P>0.05) by range treatment. Twelve-hour milk production during May, June, August, and September ranged from 11.3 to 6.8 kg and 7.6 to 3.9 kg for high and lo w producing cows, respectively. Cows with high milk production lost bo dy condition during and after the breeding season, whereas cows with l ow milk production maintained body condition during the same period. L ive weight pain of calves was greater (P<0.01) for cows with high prod uction than cows with low production but was not affected (P>0.05) by range treatment. We concluded that native and seeded ranges were of si milar nutritive value for cows with high and low milk production and t hat cows with high milk production may have greater nutrient requireme nts during late summer-early fall than what was provided by native or seeded ranges. Protein may be the primary limiting nutrient in forages during the late summer for lactating cows grazing Northern Great Plai ns range.