Autumn grazing & spring silage quality
by Jeremy Hamilton
Following a run of milder winters, will we get a colder one this year? Who knows, but it’s true that longer growing seasons do throw up some questions in managing grass crops. Spring silage grass seems to be ready really early on many farms, having grown all winter, yet harvesting it can be a challenge so early in the year. Add that first cut analyses from the past couple of years have sometimes been disappointing and it is right to ask if this winter growth is any good?
Researchers in Ireland have been studying the effect that grazing autumn grass with sheep in December has on subsequent silage quality. They divided a block of land into two, grazed and ungrazed, and completed the study over 2 full seasons to also look at annual variation. Both sides had exactly the same management (fertiliser/slurry/silage cutting, etc), apart from one half was grazed to cover of 1400kg DM/ha in December with ewe lambs, whilst the other was ungrazed. Once the cover was reduced to 1400, the sheep were removed – an important point to remember. Silage cut from the two sides were kept separate and fed to dairy cows so that yield and performance could be monitored.
The first important point was the increase in first cut yield when the land had no grazing, 0.8 ton dry matter/ha in the first year and 1 ton dry matter/ha in the second, which was quite consistent. The difference between years was likely because the first year was a milder winter. When these grasses were analysed, the protein etc. were very similar but silage produced from the grazed land tended to have higher energy values.
The findings are logical – over-wintered grass adds to bulk in the spring, but this extra ‘old’ or dead material is of lower energy than true spring growth.
The next step was a feeding trial for these silages. In both years, cows fed the higher energy silages made from winter grazed swards produced more milk (+3% increase in litres yield in Y1 and, although not significantly higher in litres, 9% on fat:protein corrected yield in Y2, indicating the complexity of how cows process the feeds we give them).
So it’s a useful study which gives quite straightforward answers – remove excess cover in December to avoid cutting in spring, so lowering your first cut energy density. However, don’t phone the sheep farmer yet as the study raised an additional question to complicate this conclusion. When the researchers considered the entire milk potential for the forages from both sides – remember the grazed side had a lower yield – then the ungrazed land produced more milk potential on a per hectare basis as the quantity increase outweighed the quality improvement.
Different farms will have different priorities in the quality versus quantity balance, and so this info is useful to inform decisions in each individual circumstance. The study also reminded us that no two years are the same and so blanket recommendations are to be avoided. Keep an open mind, look at swards and conditions and make plans and decisions from there. Sometimes sheep grazing is a useful tool to manage silage quality, and sometimes they may just be ‘field lice’ eating tons which would be more useful in a clamp the next year.