Masters Theses

Date of Award

8-1969

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Animal Science

Major Professor

C.S. Hobbs

Committee Members

D.O. Richardson, R.R. Shrode, Will T. Butts, Luther H. Keller

Abstract

Many beef cattle breeders must exercise selection of breeding stock at weaning, even though it is recognized that environmental variation, particularly differences in maternal ability of cows, tends to disguise true genetic growth potential. Previous studies have suggested that variation in condition score is an indicator of maternal differences. Hence, it appears that pre-weaning gain may be an improved predictor of post-weaning gain if it is adjusted for variation in fatness of the calves. The objectives of the study were: (1) to describe a sample of calves as weanlings and as yearlings with respect to thickness of fat cover and depths of the 1. dorsi and b. femoris muscles, (2) to study the relationships among objective measurements, subjective scores, and post-weaning performance of calves and (3) to study the relationships among weight and fatness of dams and the performance of their offspring. Selected fat and muscle depths of 76 Hereford and 67 Angus calves and their dams were measured ultrasonically at weaning and after a post-weaning period on high-roughage rations. All calves were subjectively scored for type and condition while cows were scored only for condition. Least-squares models were calculated using calf, cow or cow and calf data to predict post-weaning performance. Mean fat thicknesses over the 1. dorsi at weaning were: Angus males, 1.69 mm.; Angus females, 3.94 mm.; Hereford males, 1.35 mm.; and Hereford females, 2.43 mm. Ultrasonically measured muscle depths were larger in weanling calves than in yearlings. Possible explanations discussed included, (1) changes in muscle configuration, (2) inadequacy of point of measurement or (3) errors in ultrasonic interpretation of muscle boundaries. Pooled coefficients of correlation between condition score and ultrasonically measured fat for respective locations and age groups were; 1. dorsi, 0.41 and b. femoris, 0.28 for weanling calves; 1. dorsi, 0.52 and b. femoris, 0.48 for yearlings; and 1. dorsi, 0.74 for cows. Coefficients of multiple determination (R²) ranged from 0.16 to 0.34 for bulls and 0.02 to 0.16 for heifers when weight and condition or fat thickness were included in hypothetical least squares models to predict post-weaning gain. Including age and gain data in models produced R² values of 0.05 to 0.34 for males and 0.05 to 0.16 for females. When weight and fatness of the dams were added to the calf data, the ranges in R² values were 0,32 to 0.41, and 0,08 to 0.23, for post-weaning gain for bulls and heifers, respectively. No consistent advantage was demonstrated for either condition score or ultrasonic measurement as an indicator of fatness in cattle.

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