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Measuring Grass

25 June 2024

This article is produced as a part of the FAS Crops & Soils Bulletin. Subscribe now to receive the full report in your inbox monthly.

A small proportion of UK dairy farmers who graze cows will measure grass religiously in the growing season. Grass measuring is less common among sheep and beef producers. However, the process is relevant to the red meat sector, the management changes arising from this data can improve a farm's pasture quality, quantity and availability which are key drivers in making more profitable sheep/beef systems.

Most of us know how much hay and silage we have made in kg/dm, but it is more difficult to quantify grazed grass in kg/dm like we do with every other feed if we are not measuring grass.

Appearances can be deceptive, a field may look green, and contain the species we want to see, but if we don’t know how much it yields or understand some basic grazing principles, we could be hampering the performance of our livestock and perhaps then relying on bought-in feeds.

The grass data also offers an opportunity for estimating farm average cover predicting excess and deficits of forage allowing us to adjust stocking rate, the adage that “the difference between a good farmer and a bad farmer is two weeks” still rings true and the pasture data plotted and reviewed will reveal things faster than our eyes will. The data will provide confidence when making stocking decisions.

Photo credit: Donald Dunbar

Building a pasture history allows benchmarking of years and the performance can also be compared with other businesses as well. One method for comparing grass productivity is to use Grass Check GB, an online grass monitoring project which publishes grass growth and quality figures submitted by dairy, beef and sheep farms around the country, he added. The project has been ongoing since 2019 and is a collaboration between several industry organisations.

The cheapest option for measuring grass growth is to use a sward stick, although a plethora of options are available from handheld rising electronic platemeters to quad-trailed devices. These all determine approximate dry matter yields per hectare which helps with feed budgeting and stock management.

There are standard measurements for turning out ewes or lambs into a field and for moving them on, by using these guidelines we can greatly improve lamb growth rates and help to maintain ewe condition.

Photo credit: Daniel Stout
Photo credit: Daniel Stout

Weaned Lambs and Dry Ewe Covers

Priority of grazing should be given to weaned lambs and as a rough guide for the summer months, they will require at least 2000kg/DM/ha at 6-8cm as the grazing sweet spot if rotationally grazed. Putting lambs onto covers of no more than 2,770kgs/DM/ha to maximise growth rates in the summer months. This will equate to a grass length of an estimated 8-10cm. If lambs are put onto higher pasture covers than this the plants are out of control as there is too much stem and dead material which is less palatable and has significantly fewer nutritional benefits so will impact growth rates which we want to be averaging 250g of live weight a day at a minimum. The group should be moved on when the grass has been grazed down to 1710kgs/DM/ha and measures about 4-5cm in length as grazing past this threshold will also impact the growth rates of lambs and the sward recovery time. If set stocking and not rotational we should graze fields at 1710-2240kg/DM/ha which is 4-6cm.

Dry ewes have a lower requirement of pasture if they are in good condition such as 3 BCS for a lowland type or 2.5 BCS for a hill ewe. They could be turned into a field containing 2240kgs/DM/ha, at which point the grass will be about 6cm long.  The ewes should be removed when the measurement shows 1710kgs/DM/ha, at a grass length of 4cm. However, a lot will depend on other factors, such as planned tupping dates and whether a silage cut has previously been taken, for example.

Some farmers may graze fields down to 1450kgskgs/DM/ha this is a grass height of 3cm in summer with ewes after weaning, the management reasoning for this is to have the ewes maintained on low-quality feed for at least a week to assist in drying up. To maintain the BCS of ewes they are often put onto a leader-follower system with the ewes eating the residuals from the weaned lambs with the weaned lambs getting the top third of the grass from the paddocks in the rotation.

Grass Physiology

“When grass is grazed too short for example before the 2nd leaf stage on a tiller (below 3cm) it can impact the timing of the tiller regrowth’s by up to 85%! If the pasture is continually overgrazed the plant is repeatedly forced to use its root energy reserves without sufficient recovery time, the overall health and vigour of the plant are compromised. Over time, this can lead to a reduction in the size and depth of the root system, making the sward less resilient to stressors like drought. At this time of year, this will mean losing valuable nutrition for the flock.

Photo credit: Susan Pirie

Ideally in the summer roughly 25-30 days’ rest should be in place for each paddock in summer before grazing again. However, growing conditions will dictate the actual rest period. If Nitrogen fertiliser is applied on the grazing platform in the first half of August this can be good value for money than creep feeding if an effective response is achieved in well-managed paddocks. Measuring grass is the only accurate way to see the value of summer-applied nitrogen fertiliser. Swards with high (>30%) red and white clover content may not see a benefit of Nitrogen here- so species composition also is a factor.


In summary, a failure to manage grass by over/under grazing results in a decline in dry matter and nutritional quality which impacts ultimate profitability. Grazed grass remains the cheapest feed for sheep, and it requires regular attention, to achieve optimal flock performance.

Jack Munro, SAC Consulting

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