The incorporation of fertility indices in genetic improvement programmes

In recent years there has been considerable genetic progress in milk production. Yet, increases in yield have been accompanied by an apparent lengthening of calving intervals, days open, days to first heat and a decline in conception rates, which appears to be both at the genetic and phenotypic level. Fertility has a high relative economic value compared to production traits such as protein, making it attractive to include in a breeding programme. To do this there needs to be genetic variance in fertility. Measures of fertility calculated from service dates have a small genetic compared to phenotypic variance, hence heritability estimates are small, typically less than 5%, although coefficients of genetic variance are comparable to those of production traits. Heritabilities of commencement of luteal activity determined using progesterone profiles are generally higher, and have been reported as being from 0.16 to 0.28, which could be because of a more precise quantification of genetic variance, as management influences such as delaying insemination and heat detection rates are excluded. However, it might not be the use of progesterone profiles alone, as days to first heat observed by farm staff has a heritability of 0.15. The most efficient way to breed for improved fertility is to construct a selection index using the genetic and phenotypic parameter estimates of all traits of interest in addition to their respective economic values. Index traits for fertility could include measures such as calving interval, days open, days to first service, or days to first heat but there may also be alternative measures. Examples include traits related to energy balance, such as live weight and condition score (change), both of which have higher heritabilities than fertility measures and have genetic correlations of sufficient magnitude to make genetic progress by using them feasible. To redress the balance between fertility and production, some countries already publish genetic evaluations of fertility including: Denmark, Finland, France, Germany, Israel, The Netherlands, Norway and Sweden. Introduction The aim of most dairy genetic improvement programmes is to change the genetic merit of animals so that future generations will produce milk more efficiently under the prevailing economic and social circumstances. Breeding goals have been and continue to be mainly concerned with increasing yields of milk and its components, although there is a gradual move towards broader breeding goals. Average PTAs of animals by year of birth are a commonly used indication of the success of national breeding programme in terms of milk production. Genetic trends in various dairying countries, presented using the UK’s PIN (-0.03*PTA for milk + 0.6*PTA for fat + 4.04*PTA for protein) are shown in Figure 1. The USA was ahead of all other countries in terms of PIN until recently. The percentage of North American genes in the Irish and UK dairy populations has risen from around 50% for heifers born in 1990 to around 80% for heifers born in 1998 (Matthew Winter, personal communication). So the rise in milk yield in the UK and Ireland is partly due to breed replacement of the Friesian by North American Holstein. Yield has also increased due to improved environmental factors; i.e. better nutrition, housing, health and management etc. In Canada for example, phenotypic milk yield has increased by 160 kg per year while genetic merit increased by 90 kg per year (Canadian Dairy Network, http://www.cdn.ca/, 1998). In the UK (ADC, http://www.animaldata.co.uk/, 1999) milk yield has increased by 190 kg (3.7%) per year in Holstein heifers over the past five years and the increase in genetic merit for milk yield of bulls over the last 10 years was 90 kg (1.6%) per year. Hence, in both countries about 50% of the total progress in yield can be attributed to genetic improvement alone. Similar results were obtained in the Langhill experiment in Scotland. Yields increased by 4% per year, and half of this increase could be attributed to the difference between selected and control animals (Simm et al., 1994). Hence, milk yield is increasing by about 3-4% per year and approximately half of this improvement is due to genetic selection.