14 February 1997

STAR MILKERS NEED FUEL TO DO THE JOB…

Do high genetic merit cows need high input systems to sate their drive for milk production? If not fuelled, that drive can lead to thin cows which do not get back in calf. Sue Rider reports

TAKE anything and ask it to perform well and the demand on it will be high.

That is true of the office executive, the Grand Prix racing driver and the Olympic athlete. It is also true of the high genetic merit cow.

British scientists have shown that high merit cows are more efficient at converting food into milk, on both high forage and high concentrate systems. But research also shows that for some cows their drive to produce milk is such that they need the right quantity and quality of fuel. Limit energy consumption and their condition and fertility performance could suffer.

Evidence comes from the Scottish Agricultural Colleges Langhill dairy herd, Edinburgh (see opposite) and from ADAS Bridgets Research Centre, Martyr Worthy, Winchester, Hants.

High merit stock

Centre manager, Bridget Drew, is used to managing high merit stock. The farms Elite herd of 60 cows averages £54 PIN and many of the 550 commercial black-and-whites match this genetic level.

Dr Drew is the first to admit that high merit cows are on a knife edge, but maintains that when given enough high quality feed to sustain production potential, fertility performance and liveweight does not suffer (fig 1). Milk yields of 12,000 litres have been safely sustained over three years by the Elite herd, which is fed a high level of concentrates.

"High merit cows run on high input systems can gain weight progressively from lactation to lactation, and do so up until their fifth lactation," says Dr Drew.

In contrast, some cows of similar genetic potential in the MIDaS environmental herds, managed on a high forage system to constrain lactation yields to 6500 litres, have suffered poor fertility and body condition loss.

Dr Drew cites the example of cow 866 which gave 8500 litres as a first lactation heifer on the MIDaS system, and went on to produce 11,200kg in her second lactation.

But in that second lactation she was losing condition to such an extent that her performance was affected, she explains. The cow was moved out of the MIDaS herd and into the Elites. "Within 14 days of being on the higher input system her yield rose from 19kg to 37kg a day, and by the end of lactation she had gained weight," she says (fig 2).

The high input system suited this cow but, as Dr Drew points out, many dairy farms in the UK are committed to grass-based, high forage systems.

"We must find out whether high merit cows can be managed on diets of mainly good quality grazed grass and forage while ensuring their normal metabolic function and fertility, and adequate growth and body condition change between calvings."

Dr Drew believes these animals could be managed to produce high yields under a lower input system without compromising cow welfare, provided management can be adapted to enable the animal to consume high intakes of forage, or cows can be selected with an inherent ability to consume forage.

Improved management practices could include frequent cutting – every four to six weeks – of perennial ryegrass for silage, a leader/follower grazing system, and possibly zero grazing.

Sustaining milk yields

"It does not matter how you get the energy into these cows, if they are high merit they will milk. It is when the energy intake fails to sustain the yield – you are feeding for, say, 6500 litres and the cow produces 9000 litres – that the trouble starts."

Knowing what management is needed under high forage systems will become more important, for the genetic merit of the national herd is moving ahead quickly, explains Dr Drew. "Producers will soon have animals which they do not know how to manage under their forage-based systems. We must find out whether the very high genetic merit cow is the best option for such producers.

"It may be there is sensible balance to be struck between cashing in on improved efficiency and breeding an easy-care cow that will get back in-calf and demand little extra management.

"It is a question of knowing how far to go to best match the genetic potential of the cow to the intensity of the farming system," she suggests.

It could also be that some high genetic merit cows would be better suited to low input systems than others. These are just some of the questions she hopes further research will answer.

Dr Bridget Drew… knowing how to manage high genetic merit cows under high forage systems is becoming more and more important.

Milk yields of 12,000 litres have seen safety sustained over three years by Elite cows at ADAS Bridgets without compromising health and fertility.