SIMULATED INFLUENCE OF POSTWEANING PRODUCTION SYSTEM ON PERFORMANCE OF DIFFERENT BIOLOGICAL TYPES OF CATTLE .1. ESTIMATION OF MODEL PARAMETERS

Breed parameters for a computer model that simulated differences in th e composition bf empty-body gain of beef cattle, resulting from differ ences in postweaning level of nutrition that are not associated with e mpty BW, were estimated for 17 biological types of cattle (steers from F-1 crosses of 16 sire breeds [Hereford, Angus, Jersey, South Devon, Limousin, Simmental, Charolais, Red Poll, Brown Swiss, Gelbvieh, Maine Anjou, Chianina, Brahman, Sahiwal, Pinzgauer, and Tarentaise] mated t o Hereford and Angus dams). One value for the maximum fractional growt h rate of fat-free matter (KMAX) was estimated and used across all bre ed types. Mature fat-free matter (FFM(mat)) was estimated from data on mature cows for each of the 17 breed types. Breed type values for a f attening parameter (THETA) were estimated from growth and composition data at slaughter on steers of the 17 breed types, using the previousl y estimated constant KMAX and breed values for FFM(mat). For each bree d type, THETA values were unique for given values of KMAX, FFM(mat), a nd composition at slaughter. The results showed that THETA was most se nsitive to KMAX and had similar sensitivity to. FFM(mat) and compositi on at slaughter. Values for THETA were most sensitive for breed types with large THETA values (Chianina, Charolais, and Limousin crossbred s teers) and least sensitive for breed types with small THETA values (pu rebred Angus, crossbred Jersey, and Red Poll steers). Simulations of f our postweaning production systems using genetic parameters (THETA and FFM(mat)) for Hereford x Angus and Charolais crossbred steers showed that Hereford x Angus steers were approximately 7 to 8% fatter than Ch arolais crossbred steers in all production systems when slaughtered at the same weight.