Repeatability of Cow Efficiency, Weaning Weight and Milk Production in Angus, Charolais and Reciprocal Cross Cows

In this day of increasing population and decreasing natural resources, man must continually search for more efficient methods of producing foodstuffs. The livestock industry must continue to improve its efficiency of production to remain a profitable producer of protein for the human populace. The pork and poultry industry have increased their efficiency of production through extensive use of cereal grains which could be utilized as human foodstuffs. Beef cattle, on the other hand, have the capability to utilize high roughage feedstuffs, such as grasses or crop residues, which are unavailable for human digestion. The beef cow, in particular, spends the production period of her lifetime on forage with very little grain supplementaion. With this forage diet she must continue to grow, maintain bodyweight, reproduce and lactate sufficiently to produce and wean a calf yearly. Efficiency of production of the beef cow is difficult and expensive information to obtain and therefore information is limited. Various methods have been used to either estimate or measure cow efficiency. Vanmiddlesworth et al. used the ratio of calf weight to cow weight as an indicator of efficiency. Another method used has been to relate cow weights and reproductive ability. Kress et al. (1969) based efficiency upon cow size, calf weaning weights, estimated and actual TDN consumption and reproductive performance. Marshall et al. (1976) discussed some factors affecting efficiency. The latter defined efficiency as the ratio of total TDN intake of the cow and calf to weaning weight of the calf. Selection is the primary means available for improvement of a trait. For selection to be effective, superior animals must be identified as early and accurately as possible. Accuracy of selection is dependent upon using predictors which correctly identify the superior animals. Further, the accuracy is influenced by the number of records per animal and the level of repeatability for the trait being improved. If the repeatability of a trait is high, then reasonable accuracy can be attained by using only one record for selection. If the repeatability is low, then more than one record must be used to attain the same accuracy. The purpose of this study was to estimate the repeatability of cow efficiency to weaning and two related traits, weaning weight and milk production. Cow efficiency was defined in this study as the ratio of total TDN intake of the cow and calf to weaning weight of the calf. Further, two methods of estimation of repeatability, intraclass correlation and principal component analysis, were compared. Cows used in this study were straight bred Angus and Charolais plus reciprocal crosses produced in 1970-72