Watch: How to measure grass with a plate meter

Method

There are two main methods available for farmers thinking about taking grassland management to the next level and recording weekly grassland measurements. One involves the use of a rising plate meter to estimate the grass covers in paddocks. The other consists of cutting and weighing representative samples of grass in a paddock to also estimate cover. Both options have their own strengths and weaknesses and it usually comes down to a farmer’s preference which method they use. This week we will go through the main points to consider when using a rising plate meter. We will cover the cutting and weighing option next week. A manual plate metre consists of an aluminium plate, folding handle and mechanical counters. There are also more modern plate meters on the market with electronic counters and systems capable of automatically uploading data to grassland management technology databases. The starting price of a basic manual plate meter is in the region of €400 while more high-tech plate meter options can cost over €1,000.

The plate meter measures the height of the grass in a paddock. When it is placed on the grass the lightweight aluminium plate moves upwards and the mechanical counter records the height in half-centimetres. There is also a counter on the handle to record the number of hits you take in a paddock. Each time the plate meter is dropped you should click this counter to keep track of the hits. You should aim to take 20 to 40 hits in each paddock. The more hits in a paddock, the more accurate the estimation of grass cover. Walking in a ‘W’ or ‘X’ pattern should also help ensure the quantity of grass in a paddock is accurately represented.

Recording

For the standard plate meter method, a notebook is essential during the grass measuring walk. Make sure each paddock is numbered correctly so that covers are easily transferable when analysing the results later. On the notebook write down the number displayed on the plate meter before walking the paddock. When you have finished walking the paddock and taken over 20 hits with the plate meter, write down the new reading. To work out the height of grass in the paddock, subtract the first reading from the final reading and divide the difference by twice the number of hits taken. For example, if the first reading is 6,000 and the final reading is 6,400 then the height is 10cm (6,400 – 6,000 = 400; 400/40 = 10cm). This calculation is based on 20 hits being taken in a paddock. The process should be repeated for each paddock.

Calculations

To convert the height in centimetres to kg DM/ha, we subtract the ideal post-grazing height/residual (eg 4cm) from the height of the grass in a paddock. Multiply the figure you get by 250, because it is estimated that there is 250kg DM/cm. Therefore, if the grass height is 10cm, the cover is 1,500kg DM/ha ({10cm – 4cm} x 250kg DM/cm = 1,500kg DM/ha). To make grass management decisions, we must work out the farm’s total grass supply. At this point what we have are grass covers in kg/DM/ha on each of our fields. To work out supply, multiply the cover by the field’s area in hectares (Table 1).

Totals

Sum the totals to get a figure for amount of grass on the farm – good grass managers treat this number the same as the amount of meal in their silo. For a beef enterprise, to evaluate whether you have too much or too little grass, take your farm’s grazing stocking rate in LU/ha and multiply by 18 (kg of grass eaten by 1LU daily). On a farm with 2LU/ha, daily demand is 36kg of grass DM. Finally, work out days ahead – the amount of grazing days on the farm if growth were to stop. To do this, divide your total grass supply by daily demand. Use Table 2 to help with management decisions. In times of surplus, paddocks can be removed for silage bales, fertiliser eased back on or extra stock sourced. In times of deficit, think about spreading more nitrogen, introducing silage or meals to stock, housing stock or selling stock.