The Evaluation of Weaning Weight, 12-month Weight and 12-month Type Score in the Selection of Replacement Heifers

The beef cattle outlook at present is one which demands attention if the needs of the future are to be met. The United States Department of Agriculture has estimated a need for one-third more meat production by 1975. Such an increase is attributable only to the rising population demands and does not reflect increased purchasing power. In addition to the rising need for greater beef production, it is equally important that this beef be produced as economically as possible. The beef cattle industry is one of the major sources of rural income in South Dakota. According to the South Dakota Crop and Livestock Reporting Service, the state ranks fourth in numbers of beef cows on farms and ranches as of January 1, 1960. They also reported that sales from cattle and calves amounted to approximately 37 percent of the cash farm income (1954-1958 average). Any improvement that will result in a more effective selection program for replacement heifers could have a significant effect on the economy of the state. The need for additional meat production plus the importance of the beef cattle industry to South Dakota presents a definite challenge to beef cattle breeding research. This challenge must be met by improving the performance of economic characters among the present pool of livestock by the effective use of genetic variation. Since the usefulness and the economic value of the individual animal depends on several things, and these are not likely all to be equally important or all to be independent of each other, it becomes economically unsafe to select for any one characteristics alone. A selection index allows extra merit in one characteristics to offset slight defects in another by combining into one figure the credits and penalties given each animal according to its superiority or inferiority in each trait. Hence, an animal with unusually high merit in one trait is not likely to be culled due to a slightly low merit in another. However, by selecting for the traits singly or by setting independent culling levels, such animals might be discarded, regardless of the superiority or inferiority of their other traits. Pertinent to the effective use of the genetic variability and the construction of an efficient selection index is the estimation of genetic and environmental interrelationships of those traits that influence practical value. The correct relative weights to be placed on each characteristics in a selection index depends on its heritability and on its economic importance. There is always the danger here of making mistakes in estimating the heritability and the correct economic values of the characteristics involved. The objective of this study then is to obtain an efficient selection method for selecting replacement heifers having improved breeding value for those traits considered in the index. The traits under consideration in selecting these heifers are weaning weight, 12-month weight and 12-month type score. Some of the heifers in this study were from inbred lines, hence, the influence of inbreeding on weaning weight, 12-month weight and 12-month type score was investigated