Milk yield data on 218 lactations from composite beef cows in semi-arid range (MB) and semi-intensive pasture (BR) locations over two years were used to study the effects of year, location, cow age, sex of calf and interactions on lactation yield and lactation curve characteristics. Weigh-suckle-weigh procedures following a 12-hr separation were used to estimate 24-hr milk yield (2 times 12-hr calf weight gain). Year-location interactions were significant (P<.05) for lactation curve scale (a) and shape (k) parameters, time of peak lactation (TPL), yield at time of peak lactation (YTPL) but not (P>.05) 205-d total yield (TY). Location differences significantly affected a, k, TPL and YTPL. Sex of calf effects significantly (P<.10) affected a and TPL. Significant (P<.10) cow age effects on a, k, TPL and YTPL were mostly between 2-yr olds compared to 3- and 4-yr old cows. Year-cow age and location-sex of calf interactions were significant (P=.04) for YTPL and TPL, respectively. Evaluation of lactation curve characteristics should include an assessment of interactions involving year, location, cow age and sex of calf effects. The trait most closely related to weaning weight, TY was only affected by year and cow age effects. Therefore both of these effects, at a minimum, should be included in genetic evaluation of total lactation yield.

R. A Kemp

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume 23: Opening: plenary; technology; transfer; beef cattle; dairy cattle; pigs., , 205–208, 1998
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