At a cost of around £150/tDM to produce, silage is not a cheap forage, therefore making sure it is the best quality when it gets to the animal should be a priority.

How silage is covered, and with what, is an important component in that process because fermenting and preserving relies on creating an oxygen-free environment for bacteria to do their job, says Ceredigion-based silage expert Dave Davies.

“Good fermentation and aerobic stability depend on preventing oxygen from entering the pit and this is where I often see it go wrong because at this stage in the silage making process farmers are often rushing to just get the job finished,’’ he says.

“If you have done a good job of covering the pit the top sheet should blow up like a bouncy castle within the first couple of days because you are sealing in the carbon dioxide (CO2), it becomes very anaerobic very quickly and that stops all the aerobic processes.’’

Side sheets are essential because without them it is impossible to get a good seal between the top and sides of the clamp.

“Skimp on this bit and you will always get waste on the shoulders of the clamp because concrete walls are porous which means oxygen can pass through them easily,’’ warns Dr Davies, of Silage Solutions.

“Cover the entire wall, from top to bottom, and allow for at least two metres of excess sheet at the top so that it can overlap with the cling film barrier and top sheet.’’

Once the clamp is filled fold in the side sheet for an airtight seal.

Place gravel bags all the way down the sides, the ramp and front, to hermetically seal the clamp.

“While many farmers use gravel bags they don’t have them touching - having no gaps is important otherwise the clamp is not sealed properly and CO2 will seep out, creating a vacuum under the sheet which then sucks in air and oxygen because nature doesn’t like vacuums,’’ says Dr Davies.

The purpose of weighing down the top sheet is to maintain good density on what is the weakest part of the clamp. It will prevent oxygen from entering the clamp.

Dr Davies recommends paying particular attention to the ramp.

Because CO2 is heavier than air, if it is allowed to seep out from the bottom of the clamp it will create a vacuum that will suck oxygen in, he says.

“As well as sheeting the rest of the clamp correctly, make sure there is at least half a metre of extra silage sheet at the front of the clamp and weigh it down well all around the edge.

“If you can seal the ramp you will end up with five per cent more silage.’’

Dr Davies advises using clingfilm on top of the clamp, preferably an oxygen barrier.

“It is hugely beneficial because it folds to the contours of the pit, clings down very quickly and reduces the risk of oxygen getting in because it creates a self-sealing layer and the anaerobic conditions silage needs for an efficient fermentation process,’’ he says.

“An oxygen barrier is better because it also reduces direct oxygen permeation through the plastic sheeting, but it is more expensive than standard film so it does have a cost implication.

“It does a very good job though and there will be a return on that investment, but only if you do the rest of the job right; if you don’t you won’t see the actual benefits.’’