Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/4/2007
Publication Date: 2/15/2008
Citation: Grings, E.E., Roberts, A.J., Geary, T.W., Macneil, M.D. 2008. Milk Yield of Primiparous Beef Cows From Three Calving Systems and Varied Weaning Ages. Journal of Animal Science 86:768-779. Interpretive Summary: Milk yield of the dam is a major determinant of growth rate in suckling beef calves. Beef production systems leading to decreased milk yield are expected to result in decreased calf gains for that system, especially where forage quality or quantity may also limit calf growth. Calves suckling dams with lowered milk yield tend to eat more forage to compensate. However, it has been suggested that this only occurs under conditions of high forage quality, perhaps because of physical constraints on intake with lower quality forages. We have previously observed decreased weaning weights in calves from a late spring calving system compared to late winter and early spring systems. The current study evaluated the milk yield, body weight and body condition score changes of primiparous cows born and raised within three calving systems and weaned at two ages as calves and the impact of these factors on the subsequent growth of their calves. Season of calving and its associated management affected time of peak milk and amount of milk yield in primiparous cows, corresponding to varied weight gains in their calves. Milk yield of cows in the late spring calving system was sensitive to diet quality. Weaning heifers at 140 versus 190 days of age in the late spring calving system impacted subsequent milk production during the first year of lactation along with a tendency to impact calf weight and gain. Calf gains in the late spring calving system are affected by joint effects of milk yield of the cow and other direct environmental effects, likely a combination of forage quantity, diet quality, and weather. Understanding the impacts of calving date on amounts and patterns of milk production can aid in developing management systems to best match nutrient needs of cow-calf pairs in different calving systems.
Technical Abstract: In a 2-yr study, primiparous beef cows produced in 3 calving systems were used to study 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 days 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 fixed effects 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 × 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 × 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.