Benefits of winter sheep grazing for silage performance

Tack sheep have a “Marmite” reputation among dairy farmers – some value their benefits as grass managers and a source of income, while others only see their ability to rob cows of early spring feed.

As UK winters get warmer and grass grows late into the season, there are consequences for the nutritional quality of spring swards from a build-up of dead plant material.

Grazing consultant Chris Duller says there is a simple rule for deciding whether sheep have a place in managing that growth on a dairy farm.

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He suggests there is no place for winter sheep on farms that turn out herds in early February. For farms with heavier soils that turn out in the second or third week in March, grazing with sheep is possible, but only until the end of January.

For those who aim to cut that first grass as silage, rather than graze it, winter sheep can play a big role, he says.

“Because of the milder winters, crown rust and other grass diseases are becoming more of an issue if we carry heavy covers through the winter. A good solution is to sweep around fields with sheep.”

Grazing study

This was also a finding in a study by the Agri-Food and Biosciences Institute (Afbi).

The study, co-funded by the Department of Agriculture, Environment and Rural Affairs (Daera) in Northern Ireland and AgriSearch, looked at how using sheep to remove autumn grass affected silage quality and cow performance.

It was carried out over two years on a 5ha (12-acre) block by researchers Aimee Craig and Conrad Ferris of the dairy research team at Afbi Hillsborough.

After third-cut silage was harvested in mid-September, slurry was applied at about 37cu m/ha.

One half of the area was ungrazed, while the other half was grazed by ewe lambs in December to a target cover of 1,400kg dry matter (DM)/ha – equivalent to a post-grazing height of about 4cm.

After spring applications of slurry and fertiliser, both sections were harvested in early May and the grass ensiled in separate silos. The silages were fed to mid- and late-lactation dairy cows.

In the first year, the cows were supplemented with 8kg/day of concentrates. In the second, this was cut to 4kg/day because the cows were at a later stage in the lactation and, as a consequence, milk yields were lower.

Effect on silage yield and quality

The study found that silage yield in the grazed section was 0.8t DM/ha lower than the harvest from the ungrazed area in the first year, when the winter was mild. In the second year, when frosts delayed sward recovery, the yield in the grazed section was 1t DM/ha lower than the ungrazed area.

But while yield was down, metabolisable energy (ME) was higher in the silage from the grazed land – by 0.2MJ/kg DM in the first year, and by 0.5MJ/kg DM in the second.

“This is due in part to the higher percentage of dead plant material that was found in the ungrazed sward at the time of harvest,” says Dr Craig.

Silage DM intakes were also studied. In the first year, there was little difference because the nutritional value of the silages was similar, but cows offered silage made from the grazed sward produced 3%, or 0.8kg, more milk a day.

In the second year, cows offered the silage produced from the grazed sward consumed 1.5kg more silage DM, a 14% increase.

While this did not result in a higher milk yield, it did have an influence on butterfat content, with a 9% uplift.

Cow performance

The researchers say the improved performance of cows on the silage from the grazed swards needs to be considered within the context of the lower silage yields.

Total yield of fat and protein a hectare was higher in the ungrazed swards – 128kg/ha in year one and 218kg/ha in year two. And grazing in winter made ground conditions more difficult for spreading slurry in the spring.

“The benefits, or otherwise, of removing autumn growth herbage by grazing sheep will likely be influenced by weather conditions during the winter and spring, and their effect on subsequent sward growth and/or sward die-off,” says Dr Ferris.

In general, though, it is recommended that heavy grass covers are removed by grazing livestock in late autumn, the study concludes. However, the date when sheep are removed and residual grazing height are important factors.

As well as removing sheep by mid-December, very tight grazing should be avoided, as this is likely to dramatically slow spring growth, the study adds.

A factor to consider is nitrogen limits in nitrate vulnerable zones. Having 300 sheep on the farm for the winter is the equivalent of 12 cows, Mr Duller advises.