22 August 1997

Reversing the trend of declining fertility

There is concern fertility is being bred out of UK dairy cows. Jessica Buss investigates what can be done

SELECTING for better dairy cow fertility will help stem the decline in this trait in the UK before it falls to the unacceptable level now seen in the US.

So says Nottingham Universitys Eric Lamming, who points out that pregnancy rate in UK dairy cows is falling by about 1% every three years. Currently, only about 50% of cows calve to first or second service. But in some US states fertility is worse, with recent predictions of a 1% fall in pregnancy rate every two years and a calving average of only 40% to first or second service.

It is time to start selecting cows on fertility as well as for yield, says Prof Lamming. Yield is a heritable trait with some heritability estimates as high as 35%. He now considers that fertility is heritable, and it should be selected for.

"We know that the earlier a cow starts to cycle after calving the better her fertility, and the higher her reproductive efficiency. Onset of cyclicity is 21% heritable and so this also can be a selection trait," he says.

Cows in a university study averaged 22 days from calving to first ovulation, providing they were not suffering retained placenta or uterine infection. The normal range is 15 to 35 days, but some cows are delayed further, says Prof Lamming. Onset of cyclicity also increases by 2.2% after each calving. And trends in days to cyclicity show a seasonal effect with the period shorter – and therefore improved – in autumn and longest in summer.

Heritability of this trait was proven using an extensive database of 2000 cows, identifying a range of parameters, and if confirmed by current studies on Holstein Friesian cows, it is high enough to be worthy of selection, adds Prof Lamming. But milk progesterone testing three-times-a-week would be needed to identify onset of cyclicity accurately. The procedure identifies progesterone hormone levels which identify the stage of reproductive cycle.

"It is important to identify what the cow does, not what the herdsman imagines she does. Thats why we need hormone profiles for each cow," says Prof Lamming.

"Generally only 60% of the herd conform to the target calving index parameter by achieving adequate pregnancy rates; 30-40% fail to fit in and are sub-fertile. We believe this sub-fertility runs in families.

"Producers who try hard to get cows in calf regardless of their natural fertility status, particularly those with elite herds who sell the lower yielders that get in calf easily, are selecting for sub-fertility." However, Prof Lamming points out that there is no information to show that high yielders are less fertile. He is merely stresses that yield should not be increased at the expense of fertility. Although less fertile cows may still be worth getting back in calf to continue in the milking herd, it is better to breed herd replacements from more fertile cows, he adds.

And to help achieve better national fertility, breeding companies should monitor a selection of each bulls daughters for fertility status.

It is possible for bulls to sire over 100,000 daughters in the UK. "If you are going to exploit a bull to that extent, daughter fertility should be checked out. This could be assessed using milk progesterone tests."

&#8226 Nottingham University Cattle Fertility Research Group is funded by MDC and MAFF.n

Prof Eric Lamming: Select cows on fertility as well as yield.

IMPROVING FERTILITY

&#8226 Breed replacements from more fertile cows.

&#8226 Progesterone test to identify heat accurately.

&#8226 Progeny test bulls for fertility.

Keen to improve dairy cow fertility? Then dont miss this years Nottingham Cattle Fertility Conference – run in association with the British Cattle Veterinary Association – to be held at the universitys Sutton Bonington site on Sept 4. Details from Marilyn Prentice (0115-9516321).