SELECTIVE PREGNANCY detection in a block-calving herd can save time and money, but it can be a risky strategy and needs top-notch management to be successful.

For the past two seasons, Dorset-based Ben and Sarah King have only pregnancy diagnosed suspect cows in their herd, assuming cows which don’t return to heat are pregnant. They reckon to have saved £400-£500 a year in vet costs on their 200 cows as a result.

“When you suspect a high empty rate – more than 20% – or a problem with something such as BVD, then PD-ing all cows is worthwhile,” says Mr King. “But we were also worried about passing disease on and causing abortions.”

He thinks with more cows cycling together in a block-calving herd, heats tend to be stronger and easier to spot. Cows, therefore, are more likely to be inseminated and conceive on time.

In addition, cows having difficult calvings, discharge, or a poor ovulation history have a vet check after calving to sort out problems early. This ensures they are clean and cycling before mating. Consequently, only about 20 cows in the herd – now expanded to 250 spring-calvers – are PD-ed each year.

“From May to July we do six weeks of AI followed by a sweeper bull. We watch cows continuously for heats, using tail-paint as an aid. Any cows seen on heat in August, September and October we view as not being in-calf so we have them scanned,” explains Mr King.

Empty cows are then culled, although Mr King usually finds they would have been high on the culling list anyway, due to lameness or a high cell count.

However, it was a lack of confidence in his ability to guess pregnancy correctly which prompted Dorset Farmer Focus contributor Clyde Jones to PD all cows in both spring-calving herds he manages.

“We used to think we had a good idea which cows were in-calf, but were usually proven wrong at PD-ing. Holstein types we didn”t think would survive our low cost system would get back in calf. Others, we saw bulling and wrote off as empty would also be in calf,” he explains.

Once confident which cows are empty, Mr Jones can cull some in late lactation when forage stocks are tight or keep them and boost production.

Vet Kate Burnby of Sussex-based Stock1st Vet Services believes a selective approach will only work in herds which are on top of their fertility. “This means a 7% empty rate, calving 50% of cows within the first 21 days and 75% in 42 days, so that by 10-11 weeks you have finished calving.”

Therefore, most block-calving herds should PD all cows, she advises. “Look to PD a proportion of cows five weeks into mating to identify whether the non-return rate stacks up against the true pregnancy rate. When the difference is small – less than 5% – you are doing a good job.

“But alarm bells ring when the non-return rate is close to 75%. We know in reality pregnancy rates to a service are unlikely to be higher than 60%.”

At this stage, any cows not in-calf can be given prostaglandin to induce a heat without having to wait another three weeks. Cystic ovaries or silent heats can be treated early, allowing the cow more time to get pregnant.

Miss Burnby then recommends PD-ing the whole herd 42 days after the end of mating, at one of four strategic vet visits for block-calving herds. “The first is 30 days after calving to examine problem cows. These are more likely to develop cystic ovaries or whites and are prone to becoming problem breeders.

“The second visit is seven days before planned start of mating to examine cows not seen bulling, so they can be treated and served from day one of mating.”