Prospects and strategies for genetic improvement of the dairy potential of tropical cattle by selection.

The import of genetic material and the use of crossbreeding to improve the dairy merit of tropical cattle has been criticized for eroding the livestock genetic resources of the tropics. An alternative is genetic improvement of the indigenous cattle through selection. The objective of the present paper is to examine the feasibility of this alternative. Constraints to genetic improvement of tropical cattle through selection are discussed. Low reproductive rates and high calf mortality reduce the intensity of selection. The generation interval, which is long in cattle, is further prolonged by the late sexual maturity and the long calving intervals in most tropical breeds. The most serious constraint is, however, that the extensive milk recording schemes which support dairy cattle breeding programmes in many temperate countries are almost non-existent in the tropics. In this situation, the most realistic approach to improvement through selection is to start with a single nucleus herd (or a group of cooperating herds). Two alternative selection programmes (with and without progeny testing) for a closed herd of 500 cows are outlined. The alternative which assumed no progeny testing, i.e. selection of bulls on pedigree information only, gave the fastest genetic improvement (predicted at 36 kg/year). By distributing breeding bulls from the herd the genetic progress can be disseminated to the outside population with a time lag of about two generations. A nucleus herd can supply about 15 selected bulls per 100 cows in the herd per year, enough for a population of several thousand cows.