BREEDING STRATEGIES FOR GENETIC-IMPROVEMENT OF DAIRY-CATTLE IN ZIMBABWE .1. GENETIC EVALUATION

Strategies of selection within local populations and of importation of semen and embryos from countries with higher genetic merit are evaluated for genetic improvement of milk yield in Zimbabwe. Local programs are progeny testing in a closed population, progeny testing combined with semen importation to sire 30% of cows, progeny testing with foreign sires as sires of bulls, and a closed nucleus selection scheme using embryo transfer started from elite imported stock. Strategies based on continuous importation are imported semen for 30, 50, or 100% of cows and semen from elite foreign bulls used on local elite cows and bulls from elite imported embryos. Gene flow methods were used to estimate the genetic means for the strategies. In predicted genetic response at 25 yr, continuous importation of all semen from the superior exporting country ranks first; second is the importation of elite embryos to provide bulls for AI; and third is the closed nucleus scheme using embryo transfer. Schemes involving importation of semen to breed bulls with local progeny testing are intermediate, and the lowest responses come from semen imported to breed cows and from the closed local progeny-testing scheme. Ranking of the strategies is influenced by the genetic correlation of performance and by the initial genetic difference between Zimbabwe and the exporting country.