GENOTYPE X ENVIRONMENT INTERACTION IN HOLSTEINS.

M. van Niekerk, a student from South Africa, reported genotype x environment interaction in SA Holstein dairy cattle, clear differences being seen in genetic parameters between herds using a total mixed ration (TMR) or pasture-based system. Specifically, genotype by environment interaction for persistency of production between the two production systems were indicated with genetic correlations for the first three lactations of below 0.64 throughout. This means that animals whose progeny perform well in TMR will not necessarily perform well under pasture conditions with regard to milk production traits. Selecting sires whose progeny will perform satisfactory or superior in both or one of the production systems will thus not be accurate, as the production system used by herds are not known and thus ignored in genetic evaluations of SA Holsteins.

The same student also investigated the effects of including average rainfall of the towns nearest to the applicable herds, as a proxy for the production system used. The results obtained suggest that the genetic background responsible for the average production level and persistency over the lactations were not consistent over the different rainfall areas, again suggesting genotype x environment interaction.