27 March 1998

High merit cows may need better feed pre-calving

Improving fertility performance in dairy cows, pros and cons

of early breeding techniques in sheep and enhancing

success rates with DIYAIfor pigs are key topics in this

Breeding and Fertility special. We start with a look at an

Irish trial examining reasons for poor fertility in high genetic

merit dairy cows. Emma Penny reports

BETTER rationing pre-calving may hold the key to poor fertility performance of high genetic merit dairy cows.

Trials at Moorepark, Fermoy, Co Cork – dairy centre for Irish advisory service Teagasc – compared performance of high and medium genetic merit cattle at grass. According to researcher Pat Dillon, the difference in fertility was much greater than expected.

Medium and high genetic index in-calf heifers were bought in to Moorepark for the trial. The medium genetic index (MGI) heifers had an average PIN of £35 – above average for Irish herds – while the high genetic index (HGI) heifers had an average PIN of £80.

Although three different feeding systems were used in the trial, these had no effect on fertility – but genetic index did (table 1).

Dr Dillon found that in both years the trial ran – cows were slaughtered because of one BSE case before the third years data was obtained – the HGI animals required more services for each conception and recorded lower pregnancy rates to first and second service. They subsequently had higher infertility rates compared with MGI animals.

"The reduced fertility of the HGI cows is a concern," says Dr Dillon. "Evidence is accumulating to suggest milk production will reduce reproductive performance when the intake of energy is insufficient to meet current milk output. This results in a prolonged negative energy balance in early lactation."

Moorepark vet and researcher John Mee, pHD student Sylvia Snijders and scientists at University College, Dublin are about to start further trials looking at why HGI fertility is poorer than expected.

"If we look at drying-off – the beginning of the period relevant to subsequent fertility – the MGI cows were in better condition than the HGI animals. From that, we have to assume that the higher milk yield means increased metabolic stress and a correspondingly greater loss in body condition score," says Dr Mee.

During the period from drying off to calving, the HGI cows gained weight faster than MGI animals. But at calving, the MGI cows still had a higher body condition score, averaging 3.5 compared with the HGI animals, which averaged 3-3.25, he says (table 2).

"Between calving and PDing the HGI animals lost more weight, so at calving they had a lower body condition score, much more rapid weight swings and, therefore, more varied backfat than MGI cows."

All factors which might affect fertility such as BVD, lepto or cystic ovaries were accounted for and rejected. "These, therefore, had no effect on fertility, so it must be due to metabolic differences." Dr Mee thinks that the rapid increase in daily liveweight gain before calving increases both muscle and fat reserves in cows.

"Increasing fat content releases a hormone called leptin. There is little known about leptin in cows, but in humans and mice it is known to be a feedback mechanism and depresses appetite – this means that the animal has an unconscious desire for its body to remain the same. So when cows eat more and gain weight, the leptin produced tells the body it is getting too fat and so intake is reduced." He says leptin is also known to help breakdown fat cells. "The twin factors of cows eating less and body fat reserves being broken down may mean cows go into an increasingly negative energy balance post-calving."

Dr Mee also suggests that leptin may increase the secretion and pulsation of luteinising hormone – a pre-ovulatory hormone. "This should have a positive effect on fertility, but appears to make little difference, as both HGI and MGI animals ovulate at the same time." But the fact that both animals ovulate simultaneously may mean that because the HGI cows ovulate when they are in negative energy balance they may produce an oocyte which is not properly formed, or the stress may lead to production of a poorer quality embryo.

The leptin-linked theory is to be tested in a new trial at Moorepark. "We want to find out whether the pre-calving ration affects subsequent fertility. We are hoping to replicate the previous HGI and MGI experiment, aiming for two different levels of daily liveweight gain and for groups of cows to be in different body condition scores at calving."

Both groups will be fed good quality silage pre-calving, but while one group will receive 3kg concentrate a head, the other groups ration will comprise 75% silage and 25% straw with no concentrates. Post-calving, all cows will be on the same ration.

Dr Mee plans to scan all cows every day from seven days post-calving until they ovulate. Once they have ovulated, they will be inseminated and flushed to assess embryo quality after calving.

If cows do not respond at first ovulation, they will be super-ovulated to allow insemination and ovary collection. "We hope to detect ovulation, inseminate and flush cows for up to three cycles to assess embryo quality. Cows will be bred normally on the fourth cycle."

The research team hopes to see if the effect of pre-calving diet reduces as time goes on, but this assumes that nothing post-calving will affect fertility.

If the trial provides only negative results, Dr Mee says it will at least have progressed research. "If there is no difference between the groups then pre-calving diet obviously has little effect. Then we will have to look at whether post-calving rations are more important." &#42

Feeding system had no effect on fertility of cattle at grass in Moorepark trials, but genetic merit did. High merit animals had an infertility rate of 21-25%, while less genetically superior animals had a rate of only 6%.

HIGH MERIT FERTILISER

&#8226 Varying body condition.

&#8226 Hormone regulation?

&#8226 Poor quality embryos?