This article discusses various methods to estimate the amount of pasture available to animals.
One of the first skills you will need to learn when grazing pastures is how to assess the quantity of herbage (pasture) available to the herd. In a drylot situation the feed can be weighed, a sample taken to determine the dry matter (DM) content of this feed, and the DM offered can be calculated. When grazing is adopted alternative techniques are required to estimate how much herbage DM is presented to animals. This information is vital for planning an appropriate ration program for the herd, and for maximising the savings in feed costs that can be obtained by grazing. For grazing management purposes the total amount of feed available to ground level or herbage mass is usually measured. Alternative techniques for measuring pastures are discussed below.
1. Eye Assessment
This is the least expensive and quickest method of determining herbage or pasture mass. An eye assessment of sward height, density and composition is made by walking through the area concerned and a mental conversion to lb DM/a is made. Visual assessment is subjective and requires a reasonable knowledge of how different sward characteristics, contribute to the amount of pasture present (e.g. density of pasture plants and leafiness). Extra care needs to be exercised with mixed swards (several pasture species growing together). For example, over-estimation of pasture mass is likely to occur when the pasture contains a high proportion of broad leaf plants such as clover. To become good at eye assessment, periodic calibration of your eye estimate of pasture mass with those obtained from quadrat cuts or a measured pasture is necessary. Alternatively you can be trained by completing a farm pasture assessment with someone who is experienced with pasture appraisal.
2. Sward height
Sward height can be determined simply by a measuring rule or by a mechanical metering device. The latter, which use a falling or rising plate/disc to measure the height of the compressed sward from ground level, e less laborious and allow multiple readings to be taken more rapidly than with a ruler. For herbage mass assessment, 30-50 readings across each paddock are taken to determine the average sward height. Sward height readings are then converted into equivalent lb DM/acre using an equation that has been developed by taking a series of quadrat cuts of herbage of known sward height. For leafy pastures the relationship is usually linear and maybe something like y (lb DM/acre) = 300 + 300x (inches). This calibration equation means that a sward height (x) of 4” would have an estimated herbage mass (y) of 1500 lb DM/acre (1500 = 300 + 300*4)). The “sward height – herbage mass” relationship may change with seasons and generally is least reliable when the grasses are in the seed head stage of growth. While you could adapt a calibration equation developed for pastures that are similar to your farm, it is better that an on-farm calibration is developed that reflects the swards the measuring meter is to be used on.
The main advantage of sward height compared to herbage mass from a management point of view is that it is easily recorded and understood and it is also closely associated with grazing behaviour, animal intake and the production of both the pasture and livestock.
In most cases sward height grazing guidelines refer to “the average height of the uppermost leaves in an undistributed canopy” and is measured by a sward stick especially made for this purpose or with a simple ruler.
3. Electronic probes or capacitance meters
Electronic meters for measuring pasture mass are more expensive and prone to physical damage than height meters. However, they usually incorporate a computing capacity that allows paddock numbers, areas and pasture mass to be stored and simple calculations to be performed. This information can be downloaded a desktop computer and a larger database (e.g. to enable a comparison with previous pasture assessments). Normally, 30-50 measurements per paddock are taken by placing the probe in the sward to ground level. At each placement the capacitance of the sward is read and this measure is converted into lb DM/ac; using an internal calibration equation. In some cases an equation can be selected from the computer memory to match local conditions. However, as for the sward height meters, it is preferable that a series of quadrat cuts and readings between probe readings and pasture mass is taken to develop your calibration formula.
4. Pasture quadrat cuts
This is the slowest, most labour intensive and usually most expensive method of determining pasture mass. It cannot be routinely used for pasture assessments where there are more than 3-4 paddocks, but it does provide the standard against which the indirect methods described above are calibrated. An area of pasture inside a rectangular quadrat (1.5-3.0 ft2 with a rectangle of steel rod or wood that defines the cutting boundaries) is cut to ground level with a battery-driven clipper.
If ground conditions are wet or if there is dust through the herbage the sample should be washed thoroughly to remove any soil contamination. Placing the sample on a fine metal sieve or into a mesh bag before washing, or floating the material in a tank of water are suitable methods for removing any soil collected. The weight of DM harvested from the quadrat is determined by drying the fresh material until all the water is removed. This can be completed over a 24 hour period using an oven set to 180oF. A more rapid (c. 15 minutes) determination of DM content can be made using a microwave oven. The weight of DM harvested from the quadrat is converted to an equivalent per acre value. The number of quadrat cuts that needs to be taken for a pasture is dependent on sward uniformity – more being required where there is a lot of variation. In most cases 5-10 per paddock is adequate for grazing management purposes. If pasture cuts are being taken and a height or electronic meter is also being used, readings by this device should be recorded prior to the cuts being made to generate a calibration equation. In addition, making an eye assessment of the pasture prior to cutting will help to establish a more reliable mental calibration for visual assessment of pastures.
Regardless of the technique you use for estimating how much pasture is present, you should always take into account the amount of green versus dead plant material, the amount of leaf versus stem and seedhead material, and the proportion of clover in the sward when assessments are being made. All of these pasture quality factors affect animal performance, but are not included in the herbage mass measurement which only describes the quantity of herbage present. In general, if the amount of dead material is high the energy value of the herbage and animal intake will be lower than for a sward with less dead material. Similarly, intakes and the energy content of this will be greater for swards with high leaf to stem ratios, and high legume to grass ratios.
Sounds complicated? Some aspects may be, but you will be surprised how quickly the techniques for estimating herbage mass can be learnt and within a week or two you should be feeling confident with the technique that you have selected. Take time to score the herbage mass in each new paddock and then relate this to whether the pasture supply is increasing or decreasing.