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This item was digitized as part of the Million Books Project led by Carnegie Mellon University and supported by grants from the National Science Foundation (NSF). Cornell University coordinated the participation of land-grant and agricultural libraries in providing historical agricultural information for the digitization project; the University of Arizona Libraries, the College of Agriculture and Life Sciences, and the Office of Arid Lands Studies collaborated in the selection and provision of material for the digitization project.

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Data on 5130 registered Hereford cattle owned and managed by the San Carlos Apache Indian tribe were used to study cow productivity in terms of Most Probable Producing Ability and fertility. Analyses of variance and regression analyses were utilized to investigate sources of variation in the weaning weights of the calves and in their day of birth (which was a measure of cow fertility). Heritability estimates for various weights and gains and correlations (genetic, environmental and phenotypic) for weights, gains and MPPA were also calculated. Large year effects caused the most variation in weaning weight. Important too, were the effects of age of dam, interaction between year and age of dam and regression of weaning weight on day of birth. The repeatability of weaning weight was .25. This value was low compared to the average usually seen. Heritability estimates for weaning weight were .05 for males and .18 for females. Not much progress could be made selecting for weight at that stage. The heritability of postweaning weights were higher. The values for 20-month-weight were .46 for males and .31 for females, indicating reasonable progress could be made selecting for weight at that age. Twelve-month-weight had heritability estimates of .21 (males) and .17 (females). Since a weight loss occurred during the period between weaning and the 12-month-stage, the trait being measured may be a response to nutritional stress. The genetic correlations between weights at various stages were high. Some of the same genes are responsible for weights at various stages of growth. Correlations (genetic) between the weights and MPPA were generally low except the correlation of 12-month-weight with MPPA (.41). Day of birth measured as a trait of the calf was a reflection of gestation length. Heritability estimates for day of birth were .26 for calves of cows dry during the breeding season and .04 for the calves of lactating cows. When calculated as a trait of the cow, day of birth was considered a measure of cow fertility with a resulting heritability of .09. In evaluating overall cow productivity both MPPA and fertility must be considered jointly, perhaps in the form of a selection index or through independent culling levels.

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