Influence of continuous selection for improved milk yields and other productivity measures

Questions often arise as to which attributes may be negatively influenced by continuous selection for improved milk yields and other productivity measures. Also, the selection response may favour use of related animals bringing inbreeding into play. This is thought to be detrimental to reproduction, longevity and functional herd life, or is it? These questions were addressed for the South African Jersey breed by J. Du Toit and co-workers, their results being published in two papers in the South African Journal of Animal Science, Volume 42 of 2012, respectively on pages 38-46 and 55-62. The titles of the papers are: Correlated response in longevity from direct selection for production in the South African Jersey breed and Assessment of inbreeding depression for functional herd life in the South African Jersey breed based on level and rate of inbreeding.

Longevity or herd life is of economic importance as it affects overall profitability of milk production by reducing replacement costs and increasing the proportion mature, high-producing cows in the herd. Often dedicated selection for increased milk yield has resulted in a decline in fitness traits, and the question here was whether this includes longevity. Longevity in the study of Du Toit was defined as survival in the first three lactations from first calving to culling or death, adjusted for the effect of milk yield. For the purpose of the investigation performance and pedigree records of 245134 South African purebred Jersey cows from 2004 herds were retrieved from INTERGIS. The cows were progeny of 5364 sires and 124868 cows and were born between 1968 and 2005. The sample therefore was sufficiently sizable to well-represent the breed.

The results showed that dedicated selection for milk yield did not result in an undesirable genetic response in longevity. The results also suggest that direct selection for longevity in the SA Jersey breed is feasible but genetic progress will be slow because of low heritability of the trait and a long generation interval.  

Inbreeding will occur if related individuals are mated to advance productivity. Therefore, genetic improvement programmes normally balance genetic gain in economically important traits with increases in inbreeding levels. The undesirability of excessive inbreeding manifests in fitness traits such as health, fertility and survival, which combined affects the functional herd life. Whilst inbreeding should be negligible in large populations, it is a factor at farm level because with relatively small herds it is difficult to select only unrelated animals. The question posed in the second study of Du Toit and co-workers was to investigate the effect of inbreeding depression on functional herd life of the SA Jersey breed. For that purpose the pedigree file of 912638 individuals was obtained from the INTERGIS which included data of registered, grade and imported animals.

The results showed significant negative effects of inbreeding on functional herd life in the first two lactations, but not in the third which may be a coincidence. This provides a clear warning that the current levels or rate of inbreeding in the SA Jersey breed have reached the point that they become detrimental to functional herd life. Therefore, individual inbreeding coefficients should be considered when breeding decisions are to be made by Jersey breeders.