29 March 1996


Irish research is keen to discover what influences the daily intake of grass by dairy cows. Sue Rider reports

MAXIMISING daily intake of grass is the key to improving total milk output from grass a cow.

"Increasing daily rate of intake is the target to enable cows to reach the optimum sward height as soon as possible," says Dr Tony Brereton of the Moore-park Dairy Research Centre, Co Cork, Ireland.

His research is trying to identify what is it about the physical structure of the grass sward that limits daily grass intake. He explains that the aim is not to go below the optimum utilisation level which is about 4-6cm (1.5-2.4in) in sward height. But to get to that level faster.

"The rate of cow intake is the key because thats what maximising output from grass a cow is all about," he says.

When determining what affects grass intake, Dr Brereton says it is vital to accept that the sward is composed of layers and that those layers correspond to bite depths.

"Its been shown that when you have a tall sward a cow takes a bite which is about 12cm deep, but when shes cleaned that out her next bite isnt as deep. As she goes down the sward, her bite size gets shorter and shorter."

Bite depth decreases as the height of the sward decreases so layers decrease in depth down the sward.

Swards also comprise two phases: The long and the short. "You have clean pasture in spring. Cows go in and dung on that pasture and areas around the pats are not grazed. When cows come to graze that pasture three weeks later there will be grass grazed previously to the shorter height. Grass around the pats will be twice as long. Cows will graze that longer grass because the dung isnt fresh anymore. But it wont be grazed as tightly because its stemmier," he says. Cows fail to graze tighter than 15cm (6in) in the long phase and in the short phase stop at 5cm (2in). When the sward is shorter intakes start at about 6kg DM/ha, but fall day by day. But because the top of the long sward is so diffuse, intake is restricted to about 4kg DM/ha a day initially.

Only when the swards gets thick lower down does intake increase. "Selectivity isnt for or against dunged or undunged pasture – only for leafiness," says Dr Brereton.

He has also analysed the link between the height of the sward and its density. "Density increases progressively as you go down the swards from 50kg/ha cm at the top to 500kg/ha cm, he says.

That suggests that intakes should be getting higher as the cow goes down the sward. But because the depth of the bite is decreasing down the sward, bite volume is smaller. It is because increasing density fails to compensate for smaller bite depths as the cow bites down the sward that the actual volume of each bite decreases down the sward.

There should therefore be a limit to how high a sward should be allowed to grow. "Very tall swards will initially give poor rates of intake simply because the top gets so diffuse. At this stage a cow works hard before seeing a return on her bite."

The second factor limiting grass intakes is that cows are selective, says Dr Brereton.

Research over the last two years had shown that cows travel randomly across the pasture. In doing so they encounter patches about 1m (3.3ft) in diameter. He maintains that as the cow does her random walk encountering new patches she will bite from each unless the patch she encounters is poorer in "leafiness" than the previous one. "It would be asking a lot of the cow to be more selective than that," he says. "She merely remembers her last experience. That is true whether she took a bite at her previous encounter or not."

Initially 20% of her time is spent searching between patches. After day two it builds up to 60% and so on with over 30% time spent searching on average. "If patch size was larger and less time was spent searching we could recover 10% higher intakes," he says. That would depend on designing a sward that was more attractive to the cow and encourage intakes.

&#8226 Sward behaviour studies used 400kg Friesian cross steers gaining about 0.8kg/day. Animals were grazed on a conventional 21-28 rotation paddock system.

&#8226 Sward height.

&#8226 Sward density.

&#8226 Searching time.

Bite depth gets shorter and shorter as cows graze down the sward.

About 30% of cow grazing time is spent searching between grass patches.

Dr Tony Brereton: "Rate of intake is the key to milk output from grass."