Association between Day of Birth and Corrected Weaning Weight in Beef Cattle

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Abstract
Weaning weights of 914 Hereford calves born from 1951 through 1965, and 459 Angus calves born from 1951 through 1962 at the Fort Reno Livestock Research Station were used in this study. The analyses were restricted to calves born from January 1 to June 30. The data for each calf were day of birth, birthweight, sex, age of dam, weaning date, weaning weight and year of birth. Weaning weights were adjusted to a 205 day, mature dam, steer equivalent using standard adjustments. The associations between day of birth and adjusted weaning weights were analyzed using a regression model within each herd. The effect of year on the relation between day of birth and weaning weight was also investigated. The sums of squares removed by fitting separate curves for each year were compared with the sum of squares removed by fitting a common curve. The greatest reduction in sums of squares was due to year effect on adjusted weaning weight which was highly significant (P<.01) in both herds. The linear regression of adjusted weaning weight on day of birth common to all years was not significant in either herd. The quadratic regression of adjusted weaning weight on day of birth common to all years was significant (P<.05) in the Angus herd, but not significant in the Hereford herd. When separate regression curves for day of birth association with adjusted weaning weight were fitted for each year, significant (P<.05) regressions were present in five of the years for the Hereford herd and in two of the years for the Angus herd. These results indicate that the average association of corrected weaning weight with day of birth is small but that in some years the date of birth affects the adjusted weaning weight. Rainfall appeared to be the most probable source of environmental variation which may have caused the year by day of birth interaction. Each year when a significant (P<.05) association between day of birth and adjusted weaning weight was present, rainfall for the month of August was less than normal. In 1962 significant differences (P<.05) were present in both herds when rainfall was below normal for July and August. Calves born late in the spring appeared to have a greater reduction in their adjusted weaning weight when drought conditions occurred in mid to late summer. Copyright © 1969. American Society of Animal Science . Copyright 1969 by American Society of Animal Science

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