Articles on forages, animals, and grazing systems
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Extension Forage Specialist
University of Kentucky
Phone: (859) 257-3358
Fax: (859) 323-1952
One goal of a successful grazing system is to utilize pasture to meet animal nutritional needs for optimal performance, while maintaining a persistent, high quality stand of forage that is tolerant to grazing pressure. A second goal of managed grazing is keeping forage in a vegetative state and allowing for an adequate rest period. Rotational grazing is beneficial in many ways and may help meet these goals. When devising a rotation schedule, one of the most important factors to consider is forage availability. It seems to be a question asked often by producers. Specifically, they want to know when to move animals on and off fields. These decisions are very important because grazing affects the pasture’s persistence and quality as well as animal performance.
So, how do you estimate pasture forage availability, also referred to as forage yield or pasture mass? There are several different methods, but the most accurate form is by clipping, drying, and weighing. The first step is to clip a known area, one to three square feet staying consistent with whichever size you choose of the pasture. Cutting down to 3-4 inches above the soil, rather than at soil level, will better reflect expecting grazing height. The number of areas clipped depends on the size and variability of the field, though the recommended number of areas to cut is five to ten per paddock. Next place the clippings in a paper bag and allow the forage to dry. Most often this is accomplished by placing the forage in a forced air-oven that is set at 60-100 degrees Celsius. Once dried, the sample can be weighed to record the dry matter mass, which is then used to calculate pounds of dry matter per acre using the following equation: dry forage per square foot in grams x 100 = lbs of dry matter/acre. If you cannot dry the sample, the following equation can be used to estimate pounds of dry matter per acre: fresh forage per square foot in grams x 20 = approximate lbs of dry matter/acre. This is assuming the forage moisture content is 80 percent.
While the clipping method is accurate, it is also very time and labor intensive. This leads to fewer samples being taken and samples obtained may not represent the entire pasture well. The simplest way to measure pasture mass is by measuring forage height with a ruler or a grazing stick. Grazing sticks include a ruler and the necessary calculations to estimate grazing rotations. When using height only for a forage availability estimate, an estimate of 150-250 pounds per acre per inch of height is recommended, with 150 pounds used for thin stands and 250 pounds used for thick stands. For example, to calculate pounds of dry matter per acre, simply multiply inches of height by 250 (if a thick stand) pounds per acre-inch .If the sward height were 10 inches, then the estimated dry matter available would be 2,500 lbs. For more information on using grazing stick calculations to estimate forage availability, the publication, “Using a Grazing Stick for Pasture Management,” can be viewed by the URL http://www.ca.uky.edu/agc/pubs/agr/agr191/agr191.pdf, located on the UK Forage Extension website, www.uky.edu/Ag/Forage
Although measuring height to estimate availability is widely used, it is the least accurate because it does not account for density. Density is greatly affected by forage species and greatly impacts the amount of pasture mass. A third method that accounts for both height and density of the stand is the rising plate meter. The rising plate meter consists of a disk attached to a measuring stick, where the disk can move freely. When placing the plate meter down into the grass, the disk slides up, hence the term “rising,” as it rests on top of the grass and condenses the forage. The rising plate meter needs to be calibrated through clippings that have been dried and weighed based on the relative height of the forage being measured. However, once this calibration is done several times, an equation can be developed so that only plate meter readings will be necessary. Simply walk through the field and take several readings at random. The average height value is then used with the calibration equation to estimate forage availability. The following equation is used with the JenQuip plate meter in Missouri: Clicks(cm) x 141 + 178 = dry matter yield.
A falling plate meter can easily be constructed at home and utilizes the same principles. The following publications give instructions on building and using a plate meter, respectively: www.wvu.edu/~agexten/forglvst/fallplate.pdf and www.wvu.edu/~agexten/forglvst/passmass.pdf. While the plate meter method may be tedious at first, it proves to be much more accurate than simply measuring the height and less time consuming than clipping.
Rotational grazing systems will become more efficient with more accurate pasture availability estimations. It is worth the extra time to walk through a field before deciding to turn out animals. Put it in your weekly routine to evaluate all fields. Clipping, measuring, or taking plate meter readings do take time, but with a few seasons of experience and making observations, producers become more accurate at visually assessing each pasture quickly as they walk through. Your local county extension agent can assist you in using a grazing stick or rising plate meter, as well as help devise a pasture management plan for your farm.
A rising plate meter measures pasture mass by factoring in both plant height and density. They can be purchased through EagleDairyDirect.com/grazing services