Poor reproductive performance is a complex problem largely influenced by nutrition, health and overall cow management, but there is a genetic factor and the relatively new fertility index (FI) should be taken into account when reviewing bulls proofs and making breeding decisions.

Every bull’s Profitable Lifetime Index (£PLI) now includes a fertility component that makes up 18.5% of the overall figure, but a high £PLI does not necessarily mean a positive fertility index. Like most decisions, a balanced approach is required and it will depend on each herd’s individual circumstances how much weight is afforded to specific traits beyond the overall £PLI.

“Efficient production should still be the main selection goal on farm but there is a case for using well proven higher fertility bulls where all other selection criteria are met,” argues Nick Kirby, managing director of online semen company Bullsemen.com. “Although there is a negative link between milk yield and fertility there are bulls that buck the trend and these are the bloodlines that offer the most potential to arrest - and hopefully reverse - declining fertility.”

The extent of the problem facing in the AI industry is highlighted by the fact that only about one-third of the UK’s latest Top 100 £PLI rankings has a positive fertility index.

The fertility index, which was first introduced three years ago, is expressed as a financial figure (£) and uses calving interval and non return rates in combination with a number of predictors - including body condition scores, number of services per conception and peak yield - to produce an economic measure of daughter fertility. On average, for every £1 increase in a bull’s fertility index, there is an increase of just under half a day in his daughter’s calving interval as well as about half a percent improvement in non-return rate at 56 days.

The range between the best and worst bulls available equates to an extra 10 days in calving interval and a 10% better non-return rate. Taking the calving interval only, this potentially equates to around £50 per cow per lactation using standard industry figures on the cost of lost days.

“The genetic improvement for fertility in each generation will be relatively small but it is cumulative and is a long term solution to the problem,” says Mr Kirby.

Fertility is an even bigger issue in the US and one which AI studs are looking to address through selecting higher fertility bloodlines and promoting bulls with higher semen fertility. A Daughter Pregnancy Rate index has been available in the US since 2003 and is also included in the overall Net Merit Index.

“There is no doubt that selection for yield has depressed fertility as highlighted by USDA figures showing an average 20 day increase in calving interval over the last 40 years,” says Steve Schnell, vice president of Dairy Genetics at AI co-op CRI.

“While we can’t select just for fertility we are now actively trying to reverse this trend in our choice of sires of sons. There is evidence of the decline in fertility beginning to level out in the last 10 years, due to greater emphasis on longevity.”

In 2002 the average DPR index of the sires of sons at CRI was -1.3 compared to an average of +1.2 in 2006. This increased interest in fertility has meant that bulls in the CRI line-up like second crop bull Klassic Merrill Lynch is still one of the co-op’s best selling bulls and has been used as a sire of sons because of his good fertility rating.

High component and calving ease sire Lynch is marketed in the UK by Bullsemen.com and, with a fertility index of +7, is one of the biggest fertility improvers in the current Top 100 £PLI bulls available. With over 42,000 daughters in his proof from more than 9,000 herds, fertility traits for Lynch are 99% reliable.

Other significant high fertility index bulls include Leif, Ramos and Jardin, all Top 10 £PLI bulls and now with UK proofs. Lukas son Leif is highest ranked and has a fertility index of +4.2 and is a bull whose proof has improved quite markedly as second crop daughter data has emerged.

“There is no doubt that fertility is a big issue for both the block calving and level supply producer as extended calving intervals increase replacement and breeding costs,” says Nick Kirby. “Nevertheless, since fertility has a low heritability, a lot of daughter information is needed to produce a reliable proof so it is dangerous to use a fertility index as a primary selection tool especially when information is limited and pedigree influence is high. Whilst it is important to look for a genetic solution to fertility problems it also important to look at overall performance and to use a weighted composite index like £PLI in the UK or Net Merit dollars in the US, which includes a fertility component. In this way you will protect herd fertility without depressing production efficiency.”