1 November 1996

Grass

needs of elite cows

How should grazing management be adjusted to maximise the potential of high genetic merit cows off grass? Sue Rider reports

MANAGEMENT of high genetic merit dairy cows at pasture is a big challenge facing UK dairy farmers and one currently being addressed by researchers in Northern Ireland.

Sinclair Mayne, who is co-ordinating a team of animal and plant scientists at the Provinces Agricultural Research Institute, Hillsborough, maintains that maximising reliance on grazing with high merit cows depends on optimising grass intake.

The difficulty is that under good grazing conditions intakes of 15 or 17kg dry matter a day are normal. That is fine for the medium merit cow producing 25 litres a day, but to sustain a yield of 32.5 litres a day at grass a high index cow needs 18.7kg a day of herbage dry matter.

Dr Maynes work is, therefore, examining how to modify grazing management to accommodate the higher intake needs of top genetic cows.

The emphasis is on getting as much from grass as possible without having to resort to other, more expensive, feeds.

That is some challenge given that the cows at Hillsborough average £70 PIN and have been shown to have a marked ability to produce more milk – 37kg a day compared with 28kg a day from medium merit cows. That was achieved due to improved feed efficiency of the high merit animals, at 1.9 litres of milk/kg feed intake compared with 1.5 litres/kg of dry matter for the medium merit cows.

"High merit cows partition more feed nutrients to milk production and less to body reserves," says Dr Mayne. "This is reflected at grass."

But providing the feed nutrients required at grass to sustain milk production without compromising cow condition, fertility, or milk protein content depends on better understanding of the factors influencing grass intakes, he says.

These include the structure and texture of grass leaves, sward height, density, and grass variety. For example, evidence suggests tetraploid ryegrasses can increase intake and production relative to diploid varieties. Animal factors such as effect of milk yield on intake drive are also important.

Sward height and density are the key factors determining cow intake/bite, and, therefore, grass dry matter intake a day, says Dr Mayne. That is why it is so important to measure grass availability as kg/ha dry matter rather than sward height alone. "Intakes will vary at a given sward height depending on the density of the sward," he says.

Dr Mayne maintains that maximising the amount of grass eaten by the cow will depend on her grazing a tall 9-14cm (3.5-5.5in), dense sward. But achieving such swards could require a cut in stocking rate.

Instead of using the cow to maximise output a hectare, as in current grazing systems, the emphasis is on maximum cow intakes, even if that means a lower stocking rate.

Reducing grazing severity will reduce utilisation a hectare. More grass will be left behind and this must be controlled to prevent sward quality deteriorating, especially during spring/early summer, says Dr Mayne.

"It may be possible to graze very dense, leafy swards tightly enough to leave a shorter sward that wont deteriorate as quickly. But in most practical situations intakes will only be maximised by leaving more grass behind."

Dr Mayne hopes his research will identify the target sward density, height – and the target grazing residual – needed to optimise intakes.

He suggests rotational grazing is essential for management of high genetic merit cows at grass. Sward quality is easier to maintain under such a system. Swards can be controlled by topping or an integrated cutting for silage and grazing strategy, or a leader follower system can be implemented to use residual herbage.

In rotational grazing systems the sward is also presented to the cow, rather than expecting the cow to search the field for grass, so increasing intakes.

To produce dense swards that increase intake/bite, tillering is important, says Dr Mayne. Removing autumn herbage would help open up the base of the sward and increase tillering. Ideally, he suggests extending the grazing season and letting cows use the autumn grass. Failing that let sheep graze the pasture in late autumn.

Dr Mayne acknowledges that it may be necessary to supplement high indexing cows at grass at certain times of the year.

"But if extra feed is needed, it should be a high density concentrate and not forage, which has a higher substitution rate at grass and which cannot be targeted at those animals that will respond best to additional feed."

How can grass make the most of high genetic merit dairy cows? Its a challenge being addressed by Dr Sinclair Mayne at Northern Irelands Agricultural Research Institute, Hillsborough.


BGSWINTER MEETING

Grass and Forage for Cattle of High Genetic Merit is the theme of the British Grassland Societys winter meeting to be held on Mon and Tue, Nov 25 and 26 at the Abbey Hotel, Great Malvern, Worcs.

Full details available from the BGS office (01734-318189).


&#8226 Provide dense, tall, leafy swards.

&#8226 Adopt rotational grazing system.

&#8226 May require a drop in stocking rates.

&#8226 Supplement with high density concentrate.


FOR MORE FROM GRASS


&#8226 For more on winter feeding of high genetic merit cows, dont miss next weeks issue.