5 July 2002



Latest research data

suggests that Northern

Ireland producers face

declining cow fertility,

but some are achieving

acceptable levels, as

Richard Allison finds out

COW infertility is an increasing problem for many dairy herds in Northern Ireland. But acceptable levels of fertility can be achieved by changing herd management.

Poor herd fertility makes herd management more complex, says David Mackey, researcher at the Agricultural Research Institute of Northern Ireland, Hillsborough. "This hidden cost has most impact on herds operating a compact calving pattern to maximise milk production from grass and also increases costs in winter calving herds."

In addition, there is the cost of higher culling rates and greater semen use. But what factors affect herd fertility and is management more important than genetic merit?

To answer these questions, Dr Mackey is co-ordinating a long-term study across 19 dairy herds in Northern Ireland, including more than 2500 cows. The herds selected were representative of those in the province, in terms of size, yield level and genetic merit.

"First year results from the study revealed that overall conception rates were less than 40%. This figure is lower than anticipated and is similar to reported values in the rest of the UK."

Calving rate range

However, there was a large range in calving rates with some herds achieving an overall conception rate of up to 66%. This shows it is possible to achieve good levels of fertility.

One key factor in herds with good conception rates was managing cows to minimise body condition loss during early lactation. These herds limited body condition loss to 0.3 units during the first 100 days of lactation, half the amount lost in herds with lower conception rates, says Dr Mackey.

"Many experts believe one reason for lower conception rates is increasing cow genetic merit and milk yield. However, there was no evidence of this link in the study."

It is also widely thought that spring turnout of cows has an adverse effect on herd conception rates, but this was not found in the herds observed. Rates for February were lower than in March and April, when cows were turned out.

"But its not all bad news. Heat detection rate across the herds was high, averaging 71%. Herds which had above average heat detection rates carried out frequent heat observations, spending up to five periods daily, of at least 20 minutes duration."

Producers operating a compact block calving pattern also had higher heat detection rates. Block calving means more cows are in oestrus at the same time, allowing them to interact with others which increases the signs of standing heat, he explains.

Low conception

Given that overall conception rates were low, the average calving interval was 407 days. One herd achieved a calving interval of less than 365 days. This was achieved by combining an above average heat detection rate, 81%, with the shortest average interval to first insemination – 67 days – and the highest herd conception rate of 66%.

"This proves it is possible to have a calving interval shorter than 380 days, despite having high genetic merit cows yielding more than 7500 litres. This producer manages rations to minimise body condition loss and pays extra attention to record keeping and AI technique."

To maintain a calving interval of 365 days, cows must be inseminated earlier than the average of 84 days for first AI, as less than 40% will conceive to first service. Study results suggest that bringing first service forward by four weeks to about day 56 will not compromise conception rates, he says.

However, some producers are tempted to inseminate some late calving cows before day 56 to tighten up the calving period. But this can reduce conception rates by half. Dr Mackey advises using cheap semen when serving early, to reduce costs, as 80% will not conceive.

One consequence of poor fertility is a high culling rate. On average, cows in the study only lasted three lactations. Infertility accounted for 27% of culled animals and he believes declining fertility over recent years is the main reason for increasing culling rates in the province.

"While this study showed fertility rates are generally low, acceptable fertility can be achieved in herds with high genetic merit. But it is crucial that oestrus is detected efficiently and body condition is adequately managed."

While the target is to get more than 40% of cows intended for breeding in calf within 100 days of calving, spring calving herds should achieve 100-day in-calf rates as high as 60%, he adds. &#42

Inseminaly late calving cows before day 56 to tighten up the calving period can halve conception rates, says David Mackay.

&#8226 Poor conception rate.

&#8226 Heat detection good.

&#8226 No reduction at turnout.a

Minimising body condition score loss can reduce calving interval.

Average Range

Milk yield (litres) 7,463 4,871 -10,677

Heat detection (%) 71 53-92

Conception to 37 21-66

first service (%)

Calving interval 407 359-448 (days)

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