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Inconsistency of Partial Regression Coefficients for Variables Affecting Lamb Rate of Gain

Oklahoma Argicultural Experiment Station,² Stillwater

Abstract

Partial regression coefficients (constants) were obtained for the effects of birth weight, breed of dam, birth and rearing type and sex of lamb on average daily gain from about 50 to 90 lb. Different combinations of these variables were included in the model to test the consistency of the constants when the model was changed. The data consisted of 671 Dorset-sired lambs from the second through fifth lamb crops of grade Rambouillet and grade Rambouillet x Panama-Rambouillet ewes.

The constants obtained for birth weight changed very little within year regardless of the other variables included in the model. However, the magnitude of the constants increased each year as the ewes became older or with time from 1956 to 1959. Since age of ewe and year were confounted, the reason for the change is unknown. No significant difference in lamb rate of gain was found due to breed of dam which might be as expected since the ewes were similar in breeding.

Birth and rearing type constants changed rather drastically from year to year. The constants obtained when birth weight was omitted from the model were also different than when it was included as a variable. These changes indicate a correlation between birth weight and birth and rearing type. Birth and rearing type did not have a significant effect on gain during the first two years of this study. However, these constants were significant in the third year for all model combinations. Significance of the constants in the fourth year of this study depended on whether birth weight was included in the model.

Constants obtained for sex differences also increased as the ewes became older or with time. These constants changed with the inclusion of birth weight in the model which indicates a correlation between the two variables.

The results of this study indicate that the accuracy of adjustment factors may be improved if obtained from least squares models containing only the sources of variation for which adjustments are to be made. The effect on 50 to 90 lb. rate of gain in lambs due to birth and rearing type differences may not be very important for early maturing breeds and are inconsistent, i.e. may or may not exist in different data. Further, something (probably age of dam) caused a rather consistent time trend in the constants for birth weight and sex in their relation to the rate of gain from 50 to 90 lb. Since there were no known management changes associated with the time period involved it is suggested that age of dam influences may have caused the trends.

¹ Present address: Box 518, Paducah, Texas.

² In cooperation with Animal Husbandry Research Division, ARS, U.S.P.A.