Maximising grass quality

While high quality grazed grass has the potential to support milk yields of at least 25 litres per day at this time of the year, maintaining high grass quality consistently within dairy cow grazing systems can be a real challenge! In order to achieve high grass quality, two key targets need to be achieved. Firstly, the sward must be grazed at the correct stage, and secondly, the sward must be grazed cleanly.

The target should be to graze the sward whenever the yield of grass is 3,000 – 3,300 kg dry matter per hectare (12 to 14 cm), or whenever it comes approximately 1/3 of the way up your wellington boot. Delaying grazing until the sward has exceeded this target will reduce the quality of the grass being grazed, and make it more difficult for the cows to graze the sward cleanly. To meet this target, a rotation length of 18 - 21 days will normally be required during the period of peak growth in late April/May. If this target is achieved the sward being grazed will be mainly leaf (little stem) and as such will have a high metabolisable energy content. Post-grazing the sward is likely to be ‘greener’ at the base, and will therefore recover quickly after grazing.

Under ideal conditions, cows should graze fields to a residual sward height of 4 to 5 cm i.e. to a stage where the stubble is below the height of the toe of your wellington boot. This represents a post-grazing cover of between 1,600 – 1,800 kg dry matter per hectare. While this residual sward height can be achieved if cows enter a field when the grass cover is optimum, it becomes more difficult to achieve if cows enter a field when the cover is excessive. Achieving this post-grazing target will allow light to penetrate to the base of the sward, encourage good tiller development, and maximises the quality of the regrowth for the next grazing. While consistently meeting these targets can be difficult, especially in wet weather, tight grazing will maximise sward quality throughout the season and help maintain animal performance.

While, many farmers use a topper if the post-grazing target is not being achieved, in many cases a topper does very little to improve sward quality, with its effects being mainly cosmetic. A mower will make a much better job of removing any build up of stem at the base of the sward, provided it is set correctly and meets the target post-grazing height. Nevertheless, both ‘topping’ and ‘mowing’ are wasteful, and can be largely avoided by maintaining the targets detailed above.

Grazing high yielding cows (35 litres per cow per day +) to low residual sward heights is a real challenge. Unfortunately there is a lack of good scientific information concerning the management of high yielding cows at grass. This ‘knowledge gap’ is something that the Agri-Food and Biosciences Institute (AFBI) at Hillsborough hope to address in a new research programme in the near future.  Nevertheless, if high yielding cows are allowed to graze laxly, then a follower group of lower yielding animals must be used to achieve the required post-grazing sward cover. If the grazing area is only laxly grazed, then the resultant sward quality will be poor, and this will reduce the nutrient intake of high yielding animals at the next grazing.

Target pre- and post-grazing swards