21 November 1997

Which rotation brings best profit

for elite stock?

Researchers in Northern

Ireland are planning to

challenge conventional dairy

cow grazing wisdom in an

attempt to increase grass

intakes. Jessica Buss

reports

ARE maximum grass intakes by high genetic merit cows best achieved by a shorter 14-day grazing rotation or a longer 28-day rotation?

That is the question being posed by researchers at the Agricultural Research Institute of Northern Ireland, Hillsborough, and the Department of Agriculture for Northern Ireland (DANI).

They are planning to find out whether altering the length of the grazing rotation is a viable way of increasing grass intakes without the need to use less productive stock to follow high yielders to mop up excess grass.

The aim is to maximise sward density and bite size in order to achieve the higher cow dry matter intakes of 19-20kg at grass required to support higher milk yields with minimum reliance on concentrate supplements, explains Scott Laidlaw, DANI biologist.

"Recent trials studying one hours grazing found that sward height has an important influence on intakes. Dry matter intakes in spring when grass is 18-20cm high and when cows enter the sward hungry can be 3.5kg/hour, but fall to 2kg/hour at 7-8cm," says Dr Laidlaw.

Less dense

When swards are less dense intake is more sensitive to height, still reaching the potential of 3.5kg/hour at 18-20cm high (7-8in) but falling to 1.5kg/hour at 10cm (4in).

"We want to achieve dry matter intakes of 19-20kg a day with high yielding cows. When sward height falls too low – even for part of the day – we are not allowing her to fulfill her potential.

"In a conventional grazing system, cows enter a sward at 15cm and graze down to 6cm. So even when she goes into a paddock she cannot achieve her potential intake, and intakes reduce further when she grazes down towards a 6cm sward height."

Cow intakes are also restricted because they only graze for a maximum of 12 hours a day, adds Dr Laidlaw.

To maximise intake potential, cows must be presented with long tillers. This means leaving more grass behind after grazing so it can regrow from a tall stubble.

"In conventional grazing, with a 21-day rotation, swards are grazed down low and there is a period of regrowth when the crop does nothing because it is building its reserves.

"However, by not grazing down as hard, the sward should have more residual leaf tiller bases – speeding up regrowth and allowing a two-week grazing rotation of tall grass." But one drawback of this 14-day grazing management would be the need for a lower stocking rate, he says.

Younger leaf

Dr Laidlaw is concerned about removing younger leaf on a 14-day rotation and depleting plant reserves. This is because a leaf only contributes to the plants reserves when it is fully enlarged. And when grazing is lenient, some tillers will not be grazed at all and more stem will be left, he explains. This means grass varieties with a tall canopy, large leaves and a few big tillers – such as those currently used for conservation – may be more suitable for this type of grazing.

An alternative to a 14-day rotation is one of 28 days – not grazing too hard, but harder than in a 14-day cycle, and leaving more residual grass than a 21-day rotation and more time for regrowth.

This system would still need a lower stocking rate than a conventional rotation, he adds.

Allowing a two-week grazing rotation so cows are eating taller grass could be the way to boost daily grass intakes, according to Hillsborough research.