So the storm clouds are gathering, the weatherman got it wrong again and the RainToday app tells the sorry story of rainfall in the next half an hour. Sod’s Law states that you are halfway through the critical silage harvest so just what should you do?

The sage advice of our elders rings loud and clear “I told you not to start today lad” but that’s of no use once you have pulled the trigger and set the machines loose in the fields. All is not lost though and by some cool-headed decision making you might just avoid making an unfortunate situation into a fur-lined, ocean going disaster. So let’s take a breath and consider the situation in the calm before the storm.

Does it really matter if the silage gets wet?

What exactly is the problem with rainfall during silage making, apart from a bit of mess on the road? Well it all comes back to dry matter percentages and dilution of pH again. The DM% of your crop is critical to the quality of the resultant silage and its value as a feed. Wet silage will lose nutrients in the effluent running out of the clamp but that’s only part of the story. The water contained in the silage will effectively dilute the acids produced by the bacteria during the fermentation. This means the bacteria have to work for longer and burn up more of the crop sugars to produce enough acid to get a stable low pH. Whilst the sugars are being lost to produce extra acid, other bacteria will be degrading critical protein into ammonia so it really, really does matter if the silage gets wet.

So you’ve planned to get the silage in dry conditions but, for whatever reason, it’s not gone to plan – what can you do now?

Have a backup plan, a second plan. This is always the solution, as hoping for the best rarely gives the desired outcome. The backup plan should start to put some structure into your response to black clouds. If this all sounds a bit too much – I would argue that it’s actually just formalizing what you already do. You already look at a few spots on the windscreen and dismiss it as a bit of passing drizzle, so what I am suggesting is just putting some thresholds in place to take out some guesswork and enable you to make calm measured decisions because….

How much rain is too much rain?

The really critical decision making will occur once the harvester is rolling. If grass gets wet in the swath before you start chopping, it’s almost always better let it dry in the field and lose some value with excessive respiration rather than bring wet low DM% silage into the clamp. Once the clamp is being filled then the issues are a lot more complex. It all comes down to percentages at the end of the day and rainfall will be having a direct impact on dry matter percentage.

You will (or should) have a target dry matter percentage for the silage coming into that clamp. This figure might have been tweaked once the first few loads arrive at the clamp and a sample can be taken. If you are fortunate enough to be using on harvester NIR such as John Deere’s Harvest Lab then the driver can give you real time DM% for each load. This will let you predict where the overall clamp dry matter percentage is going but it is also something you can do without fancy harvester electronics, as long as you have a rain gauge.

If you know that half the clamp is full of silage chopped in good conditions – say at 30% DM then you can work out what a rain shower will have done as long as you know how much rain you have had. Let’s assume the other half of the crop was already rowed up ahead of the chopper and we have had 5mm of rainfall.

If we have 40 Ha (about 100 acres) of grass waiting to be picked up in 10m (30 foot) swaths we will have around 48,000m² of swath if each swath was 1.25m wide. Rainfall of 5mm on that area will be 240,000 litres of water. Now if the chopper picks up 80% of the water that fell on to the swath we will have added just over 190,000 litres to the silage. This would drop the dry matter from 30% before the rain to just over 25% for the silage coming in after rainfall. This means the clamp average will be down to around 27.5% DM. That’s probably too low so the harvest needs to stop until the crop waiting to be picked up is back to around 27-28% DM.

Once you have this sort of information, you can start to make a plan. You can make some cut off points such as:

            Half the silage in the clamp – keep going up to 2.5mm rainfall

            Three quarters in the clamp – keep going up to 5.0mm rainfall       

If the rainfall goes over your cut off points, stop harvesting and cover the clamp. No one will thank you, as it’s the least popular job in the world but it’s vital to cover the ensiled crop to maintain the dry matter and exclude oxygen. And just to make yourself extra popular, make sure the covers are properly weighed down to get close contact between the sheet and the silage.  Just keeping out the rain will only do half a job, you need to make sure the oxygen is excluded too. In some cases, farmers will cover a half filled clamp in dry ice (solid CO2) to make sure the oxygen is excluded and to prevent a boundary layer in the silage. Whilst this is a very effective practice, in today’s world CO2 is about as popular as asbestos so I wouldn’t recommend it. Time is money as everyone knows, but it’s time and money well spent. Couple of hours overtime covering a clamp is cheap compared to just a 3% loss of silage value – in the case above that might be £750 to £1000 of lost silage value.

Obviously in a standing crop such as maize or wholecrop the amount of water you will bring in during a rain event will be less than swathed forage but the same overall rules will apply. Keep an eye on the dry matter and don’t be afraid to hit the pause button. If the contactor complains, then remind them that it’s your silage they are making and it’s at least 50% of the feed stock for your enterprise. Any good silage contractor will be working with you to produce great silage.

If you want to discuss how you can plan for undesirable events with an independent consultant or to discuss any of the other aspects of silage making covered in this series – contact Jeremy Nash at Jeremy@silageconsultant.co.uk

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