Recent work on measuring fibre (NDF) digestibility has once again highlighted the difference in grass harvested for grass silage. The steep decline in digestibility of grasses as the plant matures is well known, with higher yielding leys being even easier to miss. The range in digestibility in the grass silage we have analysed is large suggesting plenty of crop is still harvested too late. Taking 30 hour in vitro digestibility used in the Cornell model as an example then the grasses from the last 2 years range from 49% to 78%. This is the difference between straw and sugarbeet. Higher values mean faster eating, more intake and more milk/growth. The opposite of this is the 240hour totally indigestible part – that which goes straight through into the slurry pit – ranges from 2.6% to 19% of the whole plant. In other words some silages have 8 times more wood than others. The difference in milk or growth potential is huge with every 1% increase in NDFd30 worth more intake and production such that many litres can soon be won or lost.
| aNDFom | NDFd30 %NDF | uNDFd240 %DM | |
| Average | 48.0 | 67.8 | 9.1 |
| Range | 37.1 – 64.9 | 49 -78 | 2.6 – 19 |
2015/16 South West grass silage sample figures
Some of the influence on this digestibility is the growing condition the plant has experienced which we cannot do much about. However this is no excuse as most of the control of this digestibility is in our hands to take the crop at the correct maturity. Don’t be too late! Grass has been growing this year given the mild winter, and many 1st cut crops are well advanced. Don’t be tempted by talking in dates – this year brings a real danger of being too late to harvest, we need to be out looking at crops and planning when to cut.
Some of the influence on this digestibility is the growing condition the plant has experienced which we cannot do much about. However this is no excuse as most of the control of this digestibility is in our hands to take the crop at the correct maturity. Don’t be too late! Grass has been growing this year given the mild winter, and many 1st cut crops are well advanced. Don’t be tempted by talking in dates – this year brings a real danger of being too late to harvest, we need to be out looking at crops and planning when to cut.
Chop Length
Maturity I’ve mentioned as affecting feed intakes of grass forages – less digestible feeds are literally less easy for cows to eat. Another large determinant of forage intake is chop length. Shorter chopped forages have higher intakes. The only issue to chopping forages too short is the potential loss of structural fibre in the rumen. However the length of chop needed for this is often over estimated. The science says 8mm counts as structural fibre with a requirement for some pieces 20-25mm or above.
Another often under estimated benefit to short chopping forages is the reduction of sorting behaviour. Sorting is made easy for cows by providing feeds with large differences in size – so long forages mixed with concentrates are asking to be sorted. Well chopped forages make uniform mixes much less likely to be sorted. In the picture above ‘nests’ in the feed are created by sorting.
So we recommend chopping grass forage to 18-20mm. With this average length there will be enough longer particles over 20mm, without getting too long so lowering intakes and risking sorting. For reference a 5 pence piece is 19mm.
Achieving the desired dry matter of 30% is important when reducing chop length. When getting dry matters higher than 30% in some circumstances chop length could be reduced further for extra intake – we would just need to look at the overall structure of the final diet, please ask if you would like advice on this.
