19 September 1997

EXPERT GUIDANCE ON USING A PLATE METER

Plate meters have increased in popularity in the UK as a grass measurement tool. New Zealand dairy consultant Paul Bird explains how they should be used and how they work

PLATE meters, pasture meters, rising plate meters, falling plate meters, are all the same device. They are a tool used to estimate the weight of pasture dry matter in a field or on a whole farm. They measure, primarily, pasture height and then through a simple formula, convert this height into kilograms of dry matter per hectare (kg DM/ha).

Plate meters are manufactured by different individuals or companies in the UK, Ireland and New Zealand. They are not all made to the same specifications.

Pasture height recorded by the plate meter is converted into kg DM/ha through a method of calibration. Grass of different heights is cut, dried and weighted. A relationship between height and weight is developed for many different heights of grass and in each season of the year. The graphs show this relationship for five different periods of the year. Pasture height after 40 readings with a plate meter is graphed against kg DM/ha. These graphs can be used as a ready reckoner while walking the farm taking measurements. This calibration is done for you but the plate meters used in the UK are at present using New Zealand calibrations which estimate the weight of grass right down to the soil. UK calibrations are under way now.

The reason why the calibration equations (lines of the graph) change is because the dry matter % changes throughout the year. Dry matter % can change from 13% to 30% in a short period due to weather conditions and stage of growth. In winter early spring DM is low compared with that in summer. Also as we move through the summer a higher proportion of the total dry matter is contained in the bottom 2 to 3cm (1in) of the sward.

Changing formulas in the year?

Some farmers change the formulas or in other words move onto different lines on the graph as the seasons change. Others, including myself, use the winter/early spring formulas all year because:

&#8226 Its simpler. The more complicated a thing is, the more you avoid it.

&#8226 Grass growth measurements can be calculated from any formulas as long as they are constant.

&#8226 Measuring average grass cover is more important in late autumn, winter and early spring. The dry matter percentage is generally low and if the pasture is well grazed the accumulation of dry matter in the base of the sward is minimal. The winter/early spring formula or graph is adequate.

&#8226 The summer formula estimates all the material in the base of the sward and a high proportion of this is not available.

Changing the formula five times over the year is complicated. However, some farmers and consultants do it because they feel it provides a more realistic picture of what is happening on the farm.

Key measurement – average grass cover and grass growth/

The plate meter is not a particularly good tool to measure daily per cow intake of grass or the quality of the grass. It is inaccurate for these purposes. The cows will indicate if they have had too much or not enough. Keep monitoring:

&#8226 The amount of grass left after grazing.

&#8226 The milk production.

&#8226 Condition score gain or loss.

The best way to use the plate is to measure every field on the farm and calculate the average grass cover over the whole farm. Do this each week and you will quickly see when grass cover is dropping or lifting. Grass growth rates can be calculated each week also.

Target average grass covers will probably be different for each farm and each part of the country. A guide is, dont let the average grass cover drop below 1800kg DM/ha of top quality grass going onto the winter, or else growth rate in February and March will be severely penalised. Turn out at an average grass cover of about 1800 to 2000kg DM/ha. Research is being conducted at present to determine more accurate targets for UK conditions.

The plate meter is not a tool for everybody. It is a tool for people who seriously want to extend their grazing season and use a form of measurement to see if they are on track.

Dont rush out and buy one straight away thinking they will tell you what to do. Assess all the methods of monitoring grass availability. Talk to other farmers measuring and get their views. Other methods include pasture height and coring paddocks on a one to 10 basis – short to long. Simply walking the farm and looking is a form of measurement. Whatever system you use the key message is you must have targets and when you have taken your measurements compare these to your targets. This then gives you the power to make intelligent grazing management decisions quickly.

Your target in the spring may be to drop paddocks out of the grazing rotation when the average grass cover rises above 2500kg DM/ha. Another aim may be that paddocks are dropped out when you can see more than five days good grazing ahead of the cows.

The plate meter is only an aid to grazing and feeding management. It is an aid to visual assessment and not a substitute. It encourages regular walking of the farm. Each day you probably walk past the silage clamp, check the milk level in the bulk tank and check on the cake silos. How often do you check your grass on the fields?

Paul Bird… plate meters are an aid to grass measurements not a substitute.

Plate meter calibrations change as dry matter % changes throughout the year – but its simpler to use winter/early spring formulas all year.

PLATE METER USES

&#8226 Estimates weight of grass DM in field or whole farm.

&#8226 It is an aid to visual assessment – not a substitute.

&#8226 Other ways of monitoring grass include pasture height.