Potential acid loading in grass silage needs a balance
Winter forage quality has been particularly inconsistent this year, with the various regions of the UK experiencing dramatically different weather during silage-making. Whilst those in the north and Scotland continue to battle wet weather, the high temperatures in late summer across many more southern areas allowed good quality 3rd cut grass silages to help fill earlier shortfalls.
“Yet despite apparently good feed value based on nutrient levels alone, particularly in the southern half of the country, many grass silages are actually suffering from low fibre levels and high potential acid-loading (PAL) scores,” states Trident technical manager Dr Michael Marsden. “There were also significant volumes of wet silage made during poor weather in the north and in Scotland, and these are also tending to be acidic.
“Silage quality is also very variable across cuts, so care needs to be taken when switching between clamps, or dealing with different cuts as ‘layers’ within the same clamp. In fact, most farms will be dealing with at least one cut of grass silage that’s highly acidic, and the risk of acidosis this winter is definitely higher than normal.”
Table 1 sets out a method for assessing a herd’s overall acidosis risk, with a total score of nine or more indicating a clear threat to cow health and productivity. Check silage analysis reports for PAL scores, and monitor cows for signs of a significant proportion of undigested grain or fibre in the dung, or air bubbles on the surface of more liquid pools of dung. All are signs of acidosis.
Table 1 – Rumen stability assessment table
|
Level of acidosis risk |
Score |
|||
|
Low (score = 1) |
Medium (score = 2) |
High (score = 3) |
||
|
Lactation yield (litres/cow) |
<6,500 |
6,500-8,000 |
>8,000 |
|
|
Dry matter intake (kg/cow/day) |
<20 |
20-23 |
>23 |
|
|
% concentrates in ration (DM basis) |
<40% |
40-60% |
>60% |
|
|
Undigested grain or fibre in dung |
Low |
Medium |
High |
|
|
Cereal/maize silage as % total forage |
<25% |
25-45% |
>45% |
|
|
Grass silage PAL* (meq/kg) |
<850 |
850-1,000 |
>1,000 |
|
|
Total score: |
||||
* PAL = Potential Acid Loading
Total score: <8 = low risk; 9-12 = high risk; >12 = very high risk.
The first step is to rebalance the ration, using sugar beet feed, soya hulls or wheatfeed to supply additional digestible fibre, and chopped wheat straw (4-6cm) to boost structural fibre. The former helps to avoid further lowering rumen pH with too much of the rapidly fermentable starch found in cereals, explains Dr Marsden, whilst the latter will promote cudding and production of saliva, the cow’s own rumen buffer.
“These acidic silages will also be relatively unpalatable and low in rumen fermentable energy, which not only affects overall energy intake, but also the supply of energy for rumen microbes to produce microbial protein. High dry matter liquid feeds like Rouxminate (a mix of wheat and sugar beet co-products) will increase palatability and boost fermentable energy supply, whilst one of the specialist banana-flavoured liquid feeds would be a good choice to mask particularly unpalatable silages and stimulate intake.
“Although rapidly fermented in the rumen, sugars tend not to produce the propionate that leads to acidosis if in excess. Just make sure there’s plenty of digestible fibre in the ration to balance the release of energy in the rumen,” he adds.
Some farms have also been getting good milk yield responses by using rumen-protected soyabean meal supplements ( SoyPass) to supply additional rumen-bypass protein, otherwise known as digestible undegraded protein (DUP). According to Dr Marsden, it’s a cost-effective way to counter any shortfall in microbial protein supply without further overloading the rumen.
“SoyPass is a much more cost-effective source of rumen-bypass protein than even straight soyabean meal, costing around 10.2p/100g DUP, compared to a price for hi-pro soyabean meal of 14.2p/100g DUP,” he states. “Another option for farmers further north is Scottish barley distillers’ feed. It’s not only highly palatable, but also contains much higher levels of DUP than most other mid-protein feeds.
“One strategy is to swap a proportion of the rapemeal in the ration for Scottish barley distillers’ feed to increase energy supply, raise the proportion of DUP in the ration and supply much-needed digestible fibre. The yeast fragments left over from the distilling process are also known to help stimulate good rumen function.”
Finally, if energy intakes are still too low, the safest way to increase the energy density of the overall ration is by adding rumen-protected fat – typically 0.250.50kg/cow/day. As long as the other nutrients are suitably balanced, adding 0.5kg of protected fat into the ration could raise overall energy intake enough to support an extra 2.02.5 litres of milk in early lactation cows.
“Milk yield is driven by energy intake, so if poor intakes and low quality silages are still holding the herd back, the rumen-protected fat products like Golden Flake, MilkBoost and Nutrilac are well worth considering,” adds Dr Marsden.
* Prices correct at the time of writing, all prices quoted are for 29t tipped bulk loads delivered on-farm within 50 miles of origin. Prices subject to change.
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