Pasture and grazing management made simple 3: herbal growth rate information

Herbage growth rate information for your farm is essential for planning grazing decisions. You need growth rate data to plan rotation lengths, calculate stocking rates and to make other management decisions such as when to apply fertilizer or feed additional supplements.

Herbage growth rate information for your farm is essential for planning grazing decisions. You need growth rate data to plan rotation lengths, calculate stocking rates and to make other management decisions such as when to apply fertilizer or feed additional supplements. In a drylot situation the tonnages and quality, and the rate at which stored feeds are used, can be measured. Pastures are different because both the quantity and quality of the forage can change significantly through time, and this will affect the way the pastures are used. It is important that good records describing the pasture are available so that the herd feeding program can be planned.

Pasture growth rates describe the average rate at which new plant material accumulates and is expressed in terms of pounds of dry matter per acre per day (lb DM/a/d). It is more correct to describe the increase in plant material mass as net herbage accumulation (NHA) since some plant material decays during the period over which growth rates are being estimated. The amount of decay will increase as the grazing interval is extended beyond about four weeks.

For effective planning of grazing, growth rates for 2-4 week intervals are required to reflect changes in seasonal conditions. More data are becoming available for different sites and pasture types in most countries where grazing is practiced. However, much of this is only for seasonal (spring, summer and fall) yields and this limits the application of these data for planning grazing. However, you can develop your own pasture growth records inexpensively over 3-4 years using one of the techniques described below.

1. Difference in herbage mass between pasture assessments

If herbage mass (see Article 2 ) is being assessed on a regular basis then an estimate of net pasture growth can be obtained from the paddocks not grazed between assessments by dividing the difference between the readings by the number of days between the measurements. For example, if the herbage mass on paddock 1 was 1000lb DM/a on May 1 and 1560lb DM/a on May 15, 560lb of DM has accumulated in 14 days to give an average net pasture growth rate of 40lb DM/a/d. An average monthly value can be obtained by completing these calculations across several paddocks.

2. Difference technique with exclusion cages

For this method 2-4 cages to exclude livestock or other wildlife grazing from an area of pasture are required, along with a lawnmower (with a catcher) or clipping shears and a quadrat (1.5 -3.0 ft2 enclosed within a rectangle formed from steel rod or wood). The site(s) selected for the cages should be representative of the pastures and soils on the farm. If a lawnmower is used the cage sizes may be 9-18 ft2, for quadrat cuts 1.5-3.0 ft2 is adequate. The herbage at the selected site is uniformly trimmed to a fixed height (2-3”) and is excluded by the cage from grazing until sufficient pasture has grown to make a cut worthwhile (2-3” over 2-4 weeks). The regrowth is then harvested to the original cutting height (make sure that this was noted down somewhere during the first trimming). The pasture material collected is dried to determine the DM production. Let’s assume - an area 2 feet by 4.5 feet (1 square yard) is mowed and that 0.25lb DM has accumulated over 30 days. The net pasture growth will be (0.25lb DM x 4840 square yards per acre)/30 days = 40lb DM/a/d.

One of the problems encountered with grazing is that pasture growth may be very different between months within a grazing season and between years. Rainfall and temperature in particular will affect the rate of growth but so will overgrazing, trampling of wet soils (plugging) and fertiliser applications. Growth rate data should, therefore, be collected over several years to obtain an average figure which can be used for planning purposes. Several years’ data will also help you to assess the variation likely to be encountered.

Computer simulation models are available to quickly estimate pasture growth rates for individual farms, providing local rainfall, and air temperature data are available. Soil temperatures at 4” would be an additional benefit. If you do not already collect this information, which is useful for making a host of other farming decisions, consider starting today. A rain gauge and thermometer are inexpensive to buy and can be located near to a route you walk each day (e.g. on the way to the barn) so that taking readings is not an inconvenience. These simple monitoring data are powerful aids to pasture managers, since both directly influence future pasture production and can explain why current production may be different to what you expected.

Collecting pasture growth rate data using the difference between paddock scorings is the simplest way to begin estimating pasture production values for your farm. You may already have pasture scoring records for previous months from which pasture growth rates can be derived. If not, why not take note of paddocks not grazed between farm walks from now on and do the few simple calculations necessary to derive pasture growth values?


Warren Parker

Warren Parker
8 articles

Chief Executive of Manaaki Whenua/Landcare Research, NZ

Former Chief Operating Officer—Science at AgResearch, NZ

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