To Graze or not to Graze Winter Swards

A 5 ha block of land had 36 cubic metres of slurry applied to it, and half of the block was left un-grazed, while the other half was grazed by ewe lambs during December until a target cover of 1,400 kg DM/ha (4 cm sward height) was obtained. Both treatments received slurry and nitrogen fertiliser the following spring and were subsequently harvested in early May. The silage from each treatment was then offered to mid/late lactation dairy cows. In year 1 cows were supplemented with 8 kg of concentrate per day and year 2 with 4 kg per day, due to their later stage of lactation and lower milk yields. 

Allowing sheep to graze the swards during December resulted in a reduction in 1st cut yields of 0.8 and 1.0 t of DM/ha in year 1 and year 2 respectively. Year 1 was a milder winter, while year 2 had a number of frosts which likely delayed sward recovery. The analysed nutritional value of both treatments was generally similar, for the exception of metabolizable energy. The ME of the ‘grazed’ treatment silage was 0.2 MJ/ kg DM higher than the ‘un-grazed’ treatment in year 1, while in year 2 the difference was 0.5 MJ/kg DM. Due in part to the small amount of dead material at the base of the un-grazed sward at harvest.

In year 1 silage DM intakes were unaffected by winter grazing management, however cows offered silage from ‘grazed’ swards produced 0.8 kg more milk per day or an increase of 0.3%. In year 2, DM intake increased by 1.5 kg per cow in the group offered silage from ‘grazed’ swards, which did not result in an increase in milk yield but did increase milk fat content by 10%. 

Improved cow performance needs to be considered within the context of the lower silage yields associated with grazed swards. In this study the fat and protein yields per ha were 128 kg and 218 kg/ha higher in year 1 and year 2 respectively with the un-grazed swards. 

The study demonstrated the response to removing autumn grass can vary considerably between years, which is most likely down to the differences in weather conditions over the course of this two year study. Neither winter was extreme, with grass on the un-grazed treatment remaining healthy over the winter. The benefits or otherwise of removing autumn growth by grazing sheep will likely be influenced by weather conditions over the winter and spring, and the impact this will have on sward growth and/or die-off. The authors do recommend heavy autumn covers should be grazed off by mid-December, but not too tightly as this could dramatically slow spring growth.