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Estimates of Age Effects on Milk Production in Hereford Cows¹

University of Georgia, Experiment 30212 and Athens 30602

Abstract

Data were collected during a 9-year period from two unrelated Hereford herds at two locations, Georgia Station (GS) at Experiment, and Central Georgia Branch Station (CGBS) at Eatonton. Heifers were bred to calve first at 3 and 2 years of age at the GS and CGBS herds, respectively. However, management of the 2-year-olds at the CGBS was not comparable to that given older cows and, therefore, the 2-year-old data were omitted. There were 398 and 820 lactation and weaning weight observations from the GS and CGBS herds, respectively. The weaning weight data were included for comparison with milk production for age of dam effects. The data were analyzed by least squares (LS) and maximum likelihood (ML) procedures.

Year-lot effects significantly influenced milk production and weaning weight in both herds. Sex of calf was not significantly associated with milk production of the calf's dam. Date of birth significantly affected milk production and weaning weight in both herds. For each day later in a 90-day winter calving period that birth occurred, daily milk production increased 0.014±0.002 and 0.010±0.002 kg and weaning weight increased 0.300±0.044 and 0.176±0.037 kg in the GS and CGBS herds, respectively. Age of dam significantly influenced milk production and weaning weight in both herds. In general, means for milk production and weaning weight increased up to 6 years of age in the GS, and up to 5 years of age in the CGBS herds before stability was reached. The LS and ML means were similar for each age of dam group within herds. This produced similar age of dam multiplicative correction factors within herds resulting from LS and ML procedures. Correction factors for milk production for 3- through 9-year-olds averaged, respectively, 1.33, 1.19, 1.09, 1.00, 1.00, 1.00 and 1.00 for the GS herd, and 1.21, 1.11, 1.00, 1.00, 1.00, 1.00 and 1.07 for the CGBS herd. Similarly, for weaning weight they averaged, respectively, 1.12, 1.07, 1.03, 1.00, 1.00, 1.00 and 1.02 for the GS herd, and 1.09, 1.03, 1.00, 1.00, 1.00, 1.00 and 1.02 for the CGBS herd. Differences in age correction factors determined from the GS and the CGBS data for milk production and weaning weight suggest that lactation number may influence milk production and weaning weight as much as age of dam at calving.

¹ Appreciation is expressed to Dr. Walter R. Harvey, Department of Dairy Science, Ohio State University, Columbus, Ohio 43210, for providing the computer program for maximum likelihood analyses and for valuable suggestions incorporated in this paper.

² Present address: Georgia Coastal Plain Experiment Station, Tifton, Georgia 31794.

³ Computer Center, Athens, Georgia 30602.

⁴ Present address : Piedmont Substation, Camp Hill, Alabama 36850.