Rising Plate Meter Equation

Background information

The rising plate meter measures forage mass and to an extent, spatial variability in an area. To measure true spatial variability, you are best to use a satellite-based pasture measurement service like ours at Pasture.io. It is quick and easy to use, and it measures compressed height as fast as a person walking across a paddock.

The basic concept behind a rising plate meter is to measure the density of the forage. The device measures this density using a weighted plate. The plate rises and falls depending on the compressed height of the forage beneath it.

The meter has a counter that counts the number of notches on its shaft as the plate moves. Each notch represents half a centimetre of height and, in some cases, a centimetre.

To use a rising plate meter, plonk it on the grassy surface. The weight of the plate causes the forage beneath it to compress, so it is essential to record the initial number of notches on the shaft. Once you record this number, subtract it from the final number of notches, and divide by two to get the bulk height of the plate.

This height feeds into the equation that you manually calculate.

With an electronic rising plate meter, they have a digital interface to measure forage mass. As you walk through the paddock taking multiple plonks (or measurements), the electronic rising plate meter automatically calculates the forage mass (KgDM/ha).

In other words, the electronic plate meter will convert the compressed height of the pasture into KgDM/ha.

Using a rising plate meter to measure forage mass is crucial in improving yields. When using the rising plate meter to monitor forage, you must understand its equations.

Understanding the equation

The rising plate meter equation has three main elements.

• Compressed forage (pasture) height
• The multiplier
• The adder

Compressed pasture height:

The compressed height is in centimetres, and this is what the rising plate meter measures. It does not measure KgDM/ha. It is the equation that converts height into KgDM/ha.

Seasonal changes in pastures, such as higher fibre content in late spring, higher dry matter content during summer, and higher water content in winter, are the underlying reasons rising plate meters fall out of calibration. Again, the plate meter measures compressed height and not dry matter content.

The multiplier:

Essentially, the rising plate meter equation is a regression equation, which means it is a line of best fit. In other words, the multiplier is the slope. The multiplier is the number that increases the dry matter yield based on the increase in compressed pasture height. For example, if the compressed height increases by 0.5-centimetre increments and the multiplier are 140. To calculate the DM of 4 centimetres of compressed height, you would divide the 4 by 0.5 to get 8 and multiply the multiplier by 140. So the equation would be 8 x 140 = 1,220 KgDM/ha.

The adder:

The equation's adder is the formula's base number to determine a dry matter yield. In other words, if the adder is 500, and if the compressed height is 0 centimetres, the resulting dry matter result would be 500 KgDM/ha.

Recapping the equation:

Compressed height is usually counted in 0.5-centimetre increments. The multiplier is the number multiplied by the compressed height in centimetres. The adder is the base amount of forage at 0 centimetres.

If you have a multiplier of 140 and an adder of 500, for each 0.5-centimetre increase in compressed height, an increase of 140 KgDM/ha is added to the 500. A half centimeter above 0 would equal 500 + 140 = 640KgDM/ha.

With the numbers from above, we can make an equation that looks like this:

compressed height x 140 + 500 = KgDM/ha

The range of the multiplier

To determine the multiplier range in a rising plate meter formula, there are some rules of thumb to make things easier.

115 when the grass is in a highly vegetative state and is growing quickly with a low dry matter content.

140 as an average across the growing season if you are not looking to change the formula (our recommendation).

186 when the pasture has a high dry matter content (i.e. the middle of summer).

Using one equation for simplicity

The name of the game is to increase your farm's sustainable profit margins. And what better way than to maximise pasture and animal performance. Now, this is where the issue of using multiple equations lies.

If you’re an astute farmer who takes regular pasture feed tests for dry matter % and other components such as neutral detergent fibre (NDF), metabolisable energy (ME). crude protein (CP), and so on. And if you then calculate the nutritional requirements of your livestock. Then you might fit the bill of changing the equation.

But before you do, let’s think about a few things and then you might think twice about introducing another level of complexity by changing the rising plate meter equation.

Changes in pasture composition

The dry matter % of pasture and other forages changes seasonally and within grazing rotations. The fluctuations can be from rain events, frosts, temperature changes, etc.

You then have the dry matter % of the different life cycles of the plant. I’m not talking about vegetative vs reproductive. We can put that into seasonal change. I’m talking about a plant that is at post-grazing compared with a plant that is at pre-grazing.

Pre and post grazing plants are both at different stages of growth and offer different nutritional profiles, which affects the measurement and the volume that your animals can consume.

Let’s explore this further.

Changes in animal requirements

Similar to humans, ruminants have a gut fill capacity. Studies have shown an association between NDF, ME, and DM content of forages to how much an animal can consume.

We then add on top the desire for the animal to graze and consume. For example, post-partum, when milk supply is in full swing and a desire for eating energy, or heavily pregnant with milk supply dry and lack of desire for eating.

What does the change in animal requirements have on rising plate meter equations?

The animal requirements directly impact the pasture residual left behind after grazing. So, we account for the impact on residual with the animal’s diet.

Summing up, by avoiding chasing your tail,

If you begin changing the formula in the rising plate meter, you essentially end up changing the base understanding of what you measured the previous week. This position means you start to chase your tail on each little change in pasture composition rather than looking at the bigger picture and for easier leavers to pull.

After all, you’re trying to protect your post grazing residual, so you don’t over or undergraze, which means your plant growth is optimised.

So, what are the leavers?

You can work on diet changes (i.e. adding more or less fodder) and stocking rate changes (i.e. drying off cows earlier or buying new stock).

Depending on your circumstances, some of these are more practical than others, but that is the gist of using one rising plate meter equation for simplicity.

If all this sounds too complicated, you can always fire up our automatic pasture measurement service and be done with the rising plate meter.

Formulas for seasonal variation (Ryegrass based swards)

The formula you use for seasonal variation comes down to which one you feel comfortable with that will yield the best plant and animal performance. There are way too many variables to offer any hard and fast equations, but here are some suggestions from TruTest and DairyNZ.

Again, the following rising plate meter equations are a guide.

Tru-Test Seasonal Variation of Formulas

According to Tru-Test, here are some seasonal variations of the rising plate meter equations that might fit your system:

Dairy NZ Southern Hemisphere Seasonal Formulas

Dairy NZ Northern Hemisphere Seasonal Formulas

Formulas for varying forage systems

The rise and fall of a forage canopy determine the forage's mass. Therefore, it is essential to understand the forage mass equations, which can help you decide how much to forage you can offer your livestock.

The rising plate meter is a simple device involving a metallic or plastic plate with a stick embedded in the middle to reinforce your understanding of how a rising plate meter works.

It is placed on the forage canopy, gently compressing its stems and leaves. A reading is taken once the plate moves up and down the meter stick.

The reading is converted into a forage yield estimate and used to determine the appropriate stocking rate.

With the understanding reinforced, you need to understand the differences between grasses, cereals, legumes, and any other forage that you want to measure. They will all offer a different reading.

Similar to using one equation for simplicity across the seasons on a somewhat monoculture or fairly uniform sward of pasture. The same principle applies in that, as a farmer, I’d suggest that you need to gauge and take rules of thumb as to the difference between what is in the forage paddock and what the rising plate meter outputs compared to your other pastures.

These rules of thumb all take experience and knowledge with understanding animal intakes and plant responses to varying grazing styles. With practice, it becomes easier.

Again, the following rising plate meter equations are a guide.

Plantain and Chicory

Studies have shown that rising plate meters can measure first-year chicory and plantain monocultures with reasonable accuracy compared to ryegrass-based pasture swards. This finding is good news for all those that love these forages.

Lucerne (Alfalfa)

A study has shown that Lucerne measurements with a rising plate meter can be reasonably accurate. The best measurements occur when the plants are shorter due to issues where the plant leaf clogs the stem and plate, thus disrupting the readings.

As lucerne stands age, the dry matter yield per height may decline due to individual plants dying. Depending on the health of your lucerne stand, this may or may not be an issue when assessing the quantity of lucerne in KgDM/a.

Grass & Legume Mix (New Zealand)

Summarising the rising plate meter equations

In this article, we looked at the ins and outs of the rising plate meter equation. We'll also discuss how you could set and forget and move into an automated pasture measurement system. The basic concept behind a rising plate meter is to measure the density of the forage. Compressed height is in centimetres, and this is what the plate meter measures. The rising plate meter formula is a regression equation, which means it is a line of best fit.

It is the equation that converts height into KgDM/ha. Seasonal pasture changes are the underlying reasons rising plate meters fall out of calibration.  Now, this is where the issue of using multiple equations lies. For simplicity, we discussed using one equation across the seasons on a monoculture or fairly uniform sward of pasture.

It can measure grass, cereals, legumes, and any other forage that you want to measure. Alongside ryegrass, lucerne chicory and plantain can be measured with a rising plate meter with reasonable accuracy.