Tuesday, May 12, 2009

Pasture Measurement

Rory Lewandowski, Extension Educator Athens County

Pasture measurement allows a grazier to determine an estimate of how much forage dry matter (DM) is available in a pasture paddock. Once forage DM is estimated, then the grazier can figure out how many animals can be grazed in that paddock for a given period of time. This is something that experienced graziers gain an eye for over time with practice. For beginning graziers pasture measurement can help to take some of the guesswork out of allocating pasture forage and it can help to reduce the slope of the learning curve associated with management intensive grazing.

To get the most benefit from pasture measurement, it must be done on a consistent basis. Think in terms of measuring pasture on a weekly basis. For the grazier serious about increasing pasture use and efficiency, weekly pasture measurement is not something that is added or squeezed in to an already busy schedule, but rather is a necessary management task. Pasture measurement done on a weekly basis will help the grazier to make decisions about how to best use pasture paddocks and over time will provide a trend of how much forage DM is being produced per week. Using this type of information can help a grazier plan the livestock rotation for periods of rapid growth as well as periods of slow growth. The trend can provide an early warning system in a drought year and help the grazier plan accordingly.

Taking a single pasture measurement in a paddock is not advisable because of the variability that typically exists in pasture growth and density across a paddock. Take 20 to 30 separate, random measurements across the paddock. The more that are taken, the more accurate your estimate of forage DM will be.

How will you decide which paddock to measure on a weekly basis? One strategy is to measure the same paddock each week. If that paddock is due to be grazed, then measure before animals are turned in and when animals are turned out. These additional measurements will give you information about how you are managing plant residual, an important part of how quickly a plant recovers from a grazing pass. Most pastures should be managed so there is 1200 to 1400 lbs of DM after animals have grazed. The other advantage of this strategy is that you will get a good idea of pasture growth rate in terms of how much DM is being added per acre each week.
Another measurement strategy is to measure a paddock that is due to be grazed each week. This will help to give you an idea about your rotation speed. If you are consistently measuring 2400 to 3000 lbs of forage DM per acre in paddocks about to be grazed, this indicates the rest period has been long enough. If forage DM is less than 2200 lbs per acre then rotation speed should be slowed down to allow the pasture more time to recover and grow.

The next question is: what is used to make pasture measurements? The most economical option is the pasture stick, which costs about $5.00 to $7.00 and is generally provided to participants of grazing schools. The current model of the pasture stick being used in Ohio has a scale to measure forage height, a scale to estimate forage density, a table of lbs of DM/inch for various pasture forage types, information about length of grazing rotations and information about calculating the lbs of DM available for grazing. The basic steps involved in using the grazing stick are:

Measure pasture height in 20-30 random areas of the paddock. Record those heights.
Add the measurements to get a total and then divide that total by the number of measurements. This will give you average pasture height.

Estimate pasture density using the dot scale on the pasture stick. This will allow you to estimate the lbs of DM per inch.

Multiply the average height (inches) by the pasture density (lbs of DM/inch). This will give you the total DM per acre in that paddock.

Subtract the amount of residual DM you want to leave in the paddock. This is plant height after grazing times the lbs of DM/inch. If you plan for a 3 to 4 inch residual, 1200-1400 lbs is about right. Consider the result of total DM minus residual DM to be the forage available for grazing.

Figure out the amount of utilizable forage. All of the forage available for grazing will not actually get grazed. There will be waste. The smaller the paddock size and the fewer days animals spend in that paddock the higher the grazing efficiency. If you are moving animals every 3-4 days use a 60% grazing efficiency to begin with. Multiply the available forage DM by the grazing efficiency expressed as a decimal. For example 1500 lbs DM x 0.60 = 900 lbs of utilizable forage DM/acre.

Figure out the livestock need in lbs of DM/day. Most livestock will consume between 2.5 to 3.0 % of their body weight in DM per day. You will need to know the average body weights of your livestock. Multiply the DM/day requirement for an individual animal by the total number of animals that will graze in that paddock. Example 30 head of sheep that average 150 lbs, consuming 2.75% of body weight in DM/day. 150 x .0275 = 4.1 lbs of DM/day. 4.1 x 30 head = 123 lbs of DM/day needed.

Figure out what the paddock can support. Divide the utilizable forage by the livestock requirement to get how many days of grazing the paddock will provide. In our example: 900 lbs of utilizable DM divided by 123 lbs of DM needed /day = 7.3. There should be about a weeks worth of grazing in this paddock.

Make adjustments based on the measurement calculations. For example if my goal is to rotate every 3-4 days and in our example we found that one acre is providing 7 days worth of grazing for 30 head of sheep, then provide about 0.5 acres every 3 days.

Another option to measure pastures is the rising plate meter. This is a simple instrument developed in New Zealand that has a counter built in. Record the beginning number on the counter, make 30 measurements and record the end number. Subtract the end number from the beginning number and divide that result by the number of measurements to get an average. Multiply this average by a conversion factor (currently 107.04) for cool season grass pastures. Use this number as the total forage DM/acre and follow the steps outlined for the pasture stick to make the remaining animal use calculations. The plate meter is quicker, but the cost of this instrument is about $450.

In the end, whatever method is used to measure pastures has value only if those measurements are used and applied to doing a better job of managing pastures.