Abstract

Annual genetic advance in merit of dairy cattle can be assessed from selection in four pathways: Bulls to breed Bulls (BB); Cows to breed Bulls (CB); Bulls to breed Cows (BC); and Cows to breed Cows. It is usually assumed that breeding merit is determined by actions of a very large number of genes, each having a small effect (infinitesimal model of inheritance). In practice, the number may be finite, with a few genes (known as QTLs) having large effects. DNA technologies allow young animals to be tested for particular QTLs using markers. Benefits of marker assisted selection were calculated for one additive QTL over a range of sizes (allele values of 0.25, 0.38, or 0.5 genetic s.d.) and initial gene frequencies (0.01, 0.10, 0.35). Testing was conducted in the BB, BC and CB pathways. Strategies were compared using the Net Present Value of the stream of productive benefits less costs for a 30 year horizon, assuming one genetic s.d. is worth $160. Superior rates of advance were sometimes achieved at greater cost than benefits. At a cost of $10 per genotype assessment it was never profitable to genotype small QTLs. The largest QTL could be profitably used to screen young bulls before progeny testing except at the lowest starting frequency. In some circumstances, selection for a QTL resulted in improved medium term response but reduced long-term response. Knowledge of the size and frequency of QTLs and a pre-defined time horizon will be essential to discriminate between profitable and costly applications of this technology.

DJ, Garrick, and RJ Spelman

Proceedings of the New Zealand Society of Animal Production, Volume 56, , 9-11, 1996