Milk yield of primiparous beef cows from three calving systems and two weaning ages

¹ USDA-ARS, Miles City, MT 59301

^* To whom correspondence should be addressed. E-mail: elaine.grings{at}ars.usda.gov.

	Abstract

Primiparous beef cows produced in 3 calving systems were used in a 2-yr completely random design study to measure milk yield throughout a 190-d lactation (2002, n = 20; 2003, n = 24 per calving system). Calving occurred in late winter (average calving date = February 4 ± 2 d), early spring (average calving date = March 30 ± 2 d), and late spring (average calving date = May 26 ± 1 d). Additionally, cows used in this study had been weaned at varied ages as calves, creating 6 dam treatments. Dam age at weaning was 140 (late spring), 190 (late winter, early spring, late spring), or 240 (late winter, early spring) d of age. Milk production was measured by weigh-suckle-weigh technique at an average of 20, 38, 55, 88, 125, 163, and 190 d in milk. Milk yield for the 190-d lactation period was calculated as area under the curve by trapezoidal summation. Data were analyzed with a model containing treatment, year, and their interaction. Orthogonal contrasts were used to separate effects when treatment was significant (P < 0.10). Total milk yield did not differ (P = 0.42) between cows in late winter and early spring systems, but cows in the late spring system tended to differ (P = 0.09) from the average of the other 2 systems. Cows in the late spring calving system had increased milk yield in 2002 and lesser milk yield in 2003 compared to the other calving systems (treatment x year interaction P < 0.001). Cows born in late spring that had been weaned at 140 d of age produced more (P = 0.05) total milk than those weaned at 190 d of age. Peak milk yield was affected (P < 0.001) by treatment and showed a treatment x year interaction (P = 0.006). Day of peak lactation differed among treatments (P = 0.002) with cows in the late winter system peaking later (P = 0.007) than early spring cows and late spring cows peaking earlier (P = 0.004) than the average of late winter and early spring cows. Average date of peak lactation was May 4 for late winter, May 31 for early spring, and July 19 for late spring systems. Calf ADG differed (P < 0.001) for the late spring system compared to the average of the late winter and early spring systems, but the relationship interacted with year (P < 0.001). Cow BW and BW change differed among treatments (P < 0.004), with much of the difference associated with amount of milk produced or timing of peak lactation. Season of calving affects milk yield of primiparous cows and ADG of their calves while grazing Northern Great Plains rangelands.

Key Words: beef cows, calving date, milk yield, rangeland