12 December 1997

Getting em to gobble down more grass

IF cows could eat an extra 0.1g of grass in every mouthful, intakes, for the average cow biting 36,000 times/day, would increase by 3.6kg dry matter a day – enabling her to eat more than 20kg DM a day.

For anyone with high merit cows and trying to maximise milk production off grazed grass, this fact is highly significant.

The modern higher indexing (£85 PIN) cow is usually larger than her medium merit (£0 PIN) contemporary so has a higher maintenance requirement, and she partitions more nutrients to milk production – producing an extra 7.5 litres of milk.

This cow would average 32.5 litres of milk a day compared with her medium merit contemporary giving 25 litres a day.

To support this extra production additional nutrients must be supplied. Instead of the 15kg DM a day needed by the medium merit cow, the high indexing animal needs to eat a massive 18.7kg DM a day – thats an extra 25% more grass.

"The cow faces an enormous challenge to harvest that amount of grass," Sinclair Mayne of the Agricultural Research Institute of Northern Ireland, Hillsborough, told delegates at a Genus conference on Breeding and Feeding the High Indexing Cow held last week at Shepton Mallet, Somerset.

Under normal grazing conditions grass intakes are 15-17kg DM a day. This is sufficient for the lower indexing animal, but the high merit cow requires more. Dr Mayne is examining how to increase grazing intake by manipulating grass management.

Feed intake is mainly determined by how effective the cow is at harvesting grass, he said. This depends on the time the cow is prepared to graze, and the rate of intake during the grazing period. High yielding cows graze for longer – up to 650-700 minutes a day – and have higher biting rates with up to 65 bites/minute than lower-yielding cows.

But the most important factor influencing total grass intake is the cows intake/bite. This averages 0.4g DM/day, giving a total grass intake of 16.8g DM/day.

Increase intake/bite by 0.1g DM/day, and intakes would increase by 3.6kg DM/day. High merit cows will bite faster and graze for longer – but there are limits to how much grass they can eat, especially as day length falls.

"To influence intake we must ensure that she takes in a good mouthful of grass every time she takes a bite," he said.

Dr Maynes research had shown that to maximise intake/bite and total grass intake/day, grazing management should focus on presenting tall, dense, leafy swards to the grazing cow.

"This can best be achieved by using rotational grazing." He acknowledged greater management expertise would be needed, but said the higher level of management was necessary to capitalise on the full potential of grazed grass with high merit cows.

"The reality is we will have to move in this direction to maintain profitability," said Dr Mayne, who stressed that some US producers with high merit cows were looking very closely at grazing due to the economic pressure they were facing in terms of reduced milk prices and increased feed costs.

Rotational grazing presents grass to the cow in the easiest-to-graze form, offers greater flexibility to cope with fluctuations in grass growth, makes it easier to assess intake, and allows better control of grass left after grazing, said Dr Mayne.

"When the sward is tall and leafy, cows will be able to eat more than 0.4g DM/bite. They will not be able to do that under set stocking." Dr Mayne suggested cows averaging more than 30 litres should be going into swards at 15-20cm (6-8in) and leaving 10cm (4in) of grass behind. Surplus grass left after grazing could be managed in three ways: Grazing by a leader/follower group, alternate grazing and cutting or topping.

Dr Mayne recognised that even under the best grazing system high genetic merit cows would require supplementation.

When grass supplies are adequate, but potential yields are more than can be supported off grass alone, he suggested feeding a concentrate rather than a forage supplement. Substitution rates were lower for concentrates and could be better targeted to meet individual cow requirements.

In mid-May a 35 litre cow would need 5kg/day and a 40-litre cow 8kg/day.

He recognised these were high levels of concentrates, but better that than offer grass silage at two to three times the cost of concentrates and 12kg of concentrates.

"There is a place for grazed grass in the diet of a 40-litre cow providing you keep the grass through the summer and top up with concentrates. Youre using less concentrate and replacing a more expensive conserved feed with grass."

Dr Mayne said the challenge now was to increase grass intake through improved management techniques and to develop grass varieties which would tolerate these laxer grazing systems.

Sue Rider

Sinclair Mayne…how can we manage grass to maximise intakes by high genetic merit cows.

INCREASINGGRASSINTAKE

&#8226 Tall, dense leafy swards.

&#8226 Rotational grazing.

&#8226 New grass varieties.