GENETIC AND ENVIRONMENTAL PARAMETERS FOR OVULATION RATE, TWINNING RATE, AND WEIGHT TRAITS IN A CATTLE POPULATION SELECTED FOR TWINNING

A project was implemented in 1981 with the objective of increasing twi nning rate in cattle. Daughters of foundation sires had twin calves at a frequency of from 8 to 13%, and foundation females had twin calves at an average frequency of 50%. Data were analyzed on twinning rate, o vulation rate, and weight traits. The h(2) of ovulation rate increased from .11 to .38 for a single estrous cycle to the mean of eight estro us cycles. From all data, h(2) for single observation of ovulation rat e and twinning rate were .10 and .09, respectively. The r(g) between t hem was .75. The h(2) of weight traits ranged from .42 to .54 when wei ght traits were analyzed pair-wise with ovulation rate and with twinni ng rate. The r(g) between weight traits with ovulation rate ranged fro m .15 to .30 and with twinning rate ranged from .24 to .39. Phenotypic mean twinning rate increased from 1.07 to 1.29 calves per parturition for females born 1981 through 1993, and adjusted mean predicted breed ing value (PBV) increased from 1.07 to 1.33 calves per parturition. Ph enotypic mean ovulation. rate for fall of 1984 through fall 1994 birth groups increased from 1.11 to 1.26 per estrous cycle, and adjusted me an PBV for ovulation rate increased from 1.11 to 1.29 per estrous. cyc le. Because of the high r(g) (i.e., .75) between ovulation rate and tw inning rate, and because of a h(2) of .35 for ovulation rate for the m ean of six estrous cycles, repeated records of ovulation rate in puber al heifers is an effective indirect selection criterion for twinning r ate. The positive r(g) between growth traits and ovulation and twinnin g rate suggest the need for some compromise when the selection goal is increased twinning rate with no increase in growth and size.