Is pre-mowing a useful tool?

As the season progresses and conditions vary, good grass management becomes increasingly important and pre-mowing can be a useful method to rejuvenate a tired pasture.

In mid-July and early August and pastures may look worked and a bit tired, with rejection sites around dung. You may well be seeing seed heads, and a higher proportion of stem to leaf than wanted, with a consequential reduction in quality. This may also be having adverse effects on intakes and causing issues with milk constituents.

Seed heads are normal this time of year. the grass plant wants to put a head up and you can’t prevent that. But, unlike Italian ryegrass, the perennial ryegrass plant only puts one head up in the year so if we remove this we can get back to leafy growth and quality.

Rejuvenating pastures can be done in a few ways:

Pre-mowing entails mowing an allocation – what the cows will realistically clear up in a feed (aiming to mow to a residual of 1,500kg DM/ha, about 5cm). Generally most people mow 8-12 hours ahead of feeding. Cows are then turned in and will clear up everything, as long as they are not given more than they can eat. They eat much that they would normally choose to leave because the wilting increases palatability. As a result we get very little to no waste, all grass cleared and a reinstated residual like a silage aftermath for good quality regrowth. 
Because cows will clear everything allocation will generally be a slightly smaller area than you would have allocated for conventional grazing. Revaluate after each feed to see how well they have done and make a decision on what to allocate at the next feed.

The conventional way to reset a paddock would be topping but this method of pre-mowing and wilting has many advantages over topping.

  • Toppers generally cut too high and shred the plant when cutting, as opposed to cutting cleanly like a mower.
  • This reduces regrowth and leaves lumps of material around the field that cows will not eat.
  • This then blocks out light beneath and impairs regrowth.
  • Animals do not fully utilise the grass – it’s a waste.
  • In a period where growth rates are slowing removing grass that the cows are not eating increases pressure because by wasting it you are reducing total grass availability. By pre-mowing and wilting you are utilising this grass, and because you are also utilising grass that would have been rejected you are increasing grass availability and decreasing pressure on the grazing block. This enables you to increase round length to take account of slower growth rates.
  • If you were going to top you would be spending money on diesel and your time so why not use that time and money to create an opportunity as opposed to waste! 

Key points for success in pre-mowing and wilting

  • When you first start the cows won’t have come across it before so may not take to it straight away. Persist as it may take a day or two for them to get on with it – accept a bit of waste and allocate a new area per feed don’t return them to what is a few days old (don’t forget once grass is cut it is starting its journey to compost!). One trick is that the cows must have an edge to their appetite when they go in.
  • Start in dry weather, make it easier for the cows. I do know of producers who use this method in wet times and one who uses this method all season and the cows clear up in the rain, but those cows know what is expected of them.
  • Allocate the right area, maybe slightly less than you would normally, so that there is grazing pressure and they are eating everything not leaving some. You may get increased dry matter intake as it is easier for cows to take bigger mouthfuls and they may eat more than a normal allocation.
  • Top covers for this method are about 3,000-3,300kg DM/ha, much more and the grass is too stemmy and there is too much for them to clear.
  • It could be argued we may have lost some potential by mowing and wilting now, rather than a few weeks ago, before seed heads really got going and grass got more fibrous as a result. Getting in earlier, as seed heads are emerging, so that we take them out and get back to good leafy regrowth earlier, is one to remember for next year. However if we follow the rules there is no reason we pre mow and wilt now as and where needed.
  • Some producers will decide to pre mow and wilt the whole round at this time of year in order to get everything reinstated. Others will pick individual paddocks that need it. That choice is a farm specific one. Make the decision by looking at the paddocks and assessing individually for residual, rejection and seed heads

Matt Rance

Whilst Matt is the technical manager of our forage services team he is also part of our ruminant technical team. Matt’s background is in dairy consultancy and practical farm management, and he joins our team to help us provide our customers with a holistic approach to feeding livestock by bringing together nutrition and innovative grassland management.