9 March 2001

DIGESTIBILITY VITAL FORPERFORMANCE

Deciding when to cut grass

and which additive to use are

crucial for optimising winter

performance of stock, as

Hannah Velten finds out

DIGESTIBILITY of grass is the most important factor affecting the feeding value of silage and consequently animal performance.

According to Tim Keady, researcher at the Agricultural Research Institute of Northern Ireland, Hillsborough, each unit increase in digestibility, lifts silage intake by 1.5%.

"Furthermore, a one unit rise in digestibility increases milk yield by 0.37kg/cow a day and milk protein by 0.016% in dairy cows and carcass gain of 28g/animal a day in beef cattle," he said.

Speaking at a Countrywide Farmers meeting in Wilts, Dr Keady said that correct date of harvest is crucial for producing a highly digestible silage with adequate crude protein (CP).

"For each week delay in harvest, digestibility is reduced by 3.6 units and maintaining performance of dairy cattle requires 1.5kg/head a day of concentrate supplementation or 1.2kg/head a day for beef cattle."

Silage has traditionally been cut at 50% ear emergence. But research at Hillsborough has shown that ear emergence is not a good guide to silage digestibility for late heading varieties.

"While there may be a three week difference in heading date between early and late varieties, to maintain animal performance the late-heading varieties must be harvested within seven to eight days of early heading harvest. Although this means harvesting late varieties with no ear emergence, possibly at a lower yield, yield will be made up in the second cut," he added.

Digestibility and CP levels are also influenced by levels of nitrogen application, he said.

"Crop lodging is a major factor reducing D-value in grass silage, by 6-9 units when severe. However, low nitrogen application will result in silage with low CP levels."

Recommendations

Current recommendations for nitrogen application – organic and inorganic – are 120kg/ha (100 units/acre) for silage grass, yet Dr Keady is concerned this is too low. He suggests nitrogen rates of 132-138kg/ha (110-115 units/acre) are applied prior to late March for a mid-May silage cut.

After grass is cut, it is essential to achieve a rapid wilt within 24 hours by spreading grass out, said Mr Keady. "Increasing dry matter from 16-30% will result in a 17% increase in intake, leading to an extra milk yield of 0.5kg/cow a day, a 0.14% rise in milk fat and a 0.11% rise in protein.

"However, when wilting on an intensely stocked farm, producers must ask where the extra 17% of silage will come from."

Prolonged wilting will increase intake but decrease digestibility and give little or no increase in animal performance. "In addition, producers should not jeopardise digestibility by prolonging harvest date in the hope of getting a wilt," he said.

DIGESTIBILITY

&#8226 Increased grass silage intake.

&#8226 Optimum nitrogen application.

&#8226 Achieve a rapid wilt.

Higher digestibility silage can increase carcass gain in growing cattle or milk yields in dairy cows, says Hillsboroughs, Tim Keady (inset).