Assessing the amount of grass available - grass cover
Grass cover is the average quantity of grass in kg DM/ha to ground level and includes the grass available for grazing and that which will be left after grazing. In this booklet grass covers are presented to ground level, and should not be confused with grass cover in the Republic of Ireland, which refers to grass available over 4 cm in height.
A number of methods have been developed to assess grass cover
Sward assessment using a rising plate meter
A rising plate meter can be used to measure grass covers until sufficient experience is gained by visual assessment. It relates pasture height and density to yield through a carefully calibrated equation.
The rising plate meter consists of a thin aluminium plate connected to a shaft by a gear linked to a read out of grass height. A mechanical counter records the number of readings from an area.
As the rod is lowered into the pasture, the plate is supported at a height determined by the sward’s density and height. A procedure for using the rising plate meter and calculating average farm cover is outlined at the end of this chapter.
Target grass covers are set pre- and post-grazing for various times during the grazing calendar and for different types of stock. Refer to the various livestock sections for more information.
Visual assessment by walking the pastures
Assessing the sward height alone is not a true reflection of grass yield. When visually assessing swards or ‘eyeballing’ as it is often called, sward height and density must be considered. Dense swards, for example a ryegrass/clover sward grazed regularly by sheep, will contain a greater amount of grass than an erect open sward grazed by cattle at the same height.
Estimating grass cover in a field can be difficult when swards are grazed unevenly as a result of poaching, spoilage and/or contamination with urine or dung. However with experience an overall average can be assigned to the field.
Calculating grass supply
Grass supply or available grass cover is the amount of grass various groups of stock are offered.
To calculate how much grass is on offer and available to the stock the estimated quantity of grass left after grazing (post-grazing) must be subtracted from the total measured amount of grass in the field before grazing (pre-grazing). This is explained in the following calculation.
grass supply or “available grass cover”.
= (pre-grazing cover – post-grazing cover) x grazing area (ha)
Typical pre-grazing and post-grazing covers for different classes of livestock in the springtime are highlighted in the Table below. In order to maintain high grass quality it is important to ensure stock enter and leave the grazing areas at the target grass covers.
|Pre-grazing KG DM/ha||Post-grazing KG DM/ha||Available grass KG DM/ha|
Stock demand for grass
The individual animal grass demand (KG DM/day) is estimated to be equivalent to 2.5 percent of the animals‘ liveweight in the case of beef cattle.
Individual animal demand can vary from 1.75 percent to 3.25 percent of bodyweight, depending on stage of growth. However, 2.5 percent has been found to be an acceptable average figure for calculation purposes.
For example, a 300 KG steer will have a daily grass demand of:
300 x 0.025 = 7.5 KG DM/day
In the case of March lambing ewes suckling 1.5 lambs, intake is estimated at 3.5 percent of bodyweight.
A ewe weighing 70 KG will have a daily grass demand of:
70 x 0.035 = 2.45 KG DM/day
The daily grass demand of a dairy cow will depend on size, milk yield and concentrate feed level.
The group demand per day is calculated by multiplying the number of animals in the group by the individual animal demand. It is this information that allows the length of the grazing period in a particular paddock or area to be calculated.
The latest grass growth rate and quality data from sites across Northern Ireland. Given the success of the ‘Grazing Management Focus’ that revolved around the GrassCheck farms in previous years, this is being extended in 2015. For more information, please visit our GrassCheck and CloverCheck section.
Budgeting grass to meet demand
Information on grass growth, grass cover (supply) and stock grass intake (demand) may then be used to prepare a grass budget. A grass budget is simply a statement of grass supply and grass demand for the grazing stock.
Grass growth predictions such as those from GrassCheck can also be incorporated to allow you to forward budget grass supply. These variations in grass growth during the year require seasonal adjustment in stocking rates to ensure efficient grassland utilisation.
Grass budgeting is particularly useful over a two to three month period in spring and autumn when grass growth rates are increasing or declining quite rapidly, or at any period when stock demand is changing significantly.
Developing a grass wedge through rotationally grazing around a number of paddocks or fields is a useful grassland management system. The grass wedge illustrates the quantity of grass available across the grazing platform. Areas are rested between grazings, allowing grass time to regrow.
Those grazing areas with the longest rest periods can be assessed weekly to identify when in the rotation grass surpluses or deficits are likely to occur. A line drawn from the target pre-grazing cover to the target post-grazing cover gives a guide to surpluses and deficits.
Establishing a grass wedge by mid-April through turn-out from early March onwards will provide a breakdown of the grass available in each paddock on the farm. To build the wedge the fields due to be grazed first in the spring should be the first to be closed off in the autumn.
Supplementation with concentrate or silage (possibly from round bales made during a grass surplus) will ensure dietary requirements are met. The grazing area could be increased by bringing in silage aftermath or removing some stock from the grazing platform.
Projecting the grass wedge using GrassCheck
A decision support tool is currently being developed combining information from the estimated grass wedge on the farm and predicted growth rates from GrassCheck.
The aim is to assist farmers with the projection of future surplus or deficits of grass within the grazing rotation, and help in the decision making process.
A practical on farm approach to this process is the concept of ‘grazing days ahead’.
Farmers who are experienced in grass budgeting can predict the grazing days ahead for a group of livestock by examining the grass wedge produced on their farm, determining the daily feed demand and incorporating growth information from GrassCheck.
Decisions can be made on whether to remove surplus grass from the grazing area based on the number of grazing days ahead of stock. For example during May, 10-12 days ahead is adequate, over 14 days will require remedial action to avoid a surplus.
Less than 10 days indicates an emerging scarcity, which requires one or more of the following options: slowing down the rotation; introducing supplementary feed; grazing some of the silage ground or reducing the stocking rate. As growth rates decline into the autumn time the target grazing days ahead of livestock should increase (25-30 days) to ensure sufficient grass is available.
Pre-grazing covers should also increase into the autumn to allow for a reduction in the quality of grass and the time spent grazing. If this is not possible then some concentrate supplementation may be necessary to maintain livestock performance.
|Pre-grazing KG DM/ha||Post-grazing KG Dm/ha||Available grass KG DM/ha|
Flexibility is key to any good grassland management plan, as changes are made throughout the grazing season to cater for periods of grass surplus and shortage.
This is made easier if the grazing area can integrate with the silage making area, which will provide grazing in both the early part of the season and in the back end when grass growth is slow.
Silage fields that are to be grazed early in the springtime should be closed off after being eaten down to approximately 1600 KG DM/ha.
Fields allocated for silage should not be grazed after the first week in April to avoid making stemmy silage.
How to use a rising plate meter
Set the top counter of the rising plate meter to 0 and record the opening reading (A) on the rising plate meter before you start.
Walk through the sward in a ‘W’ pattern, taking up to 40 measurements at equal distances apart (for example, every five steps), to ensure a uniform distribution of the sward is measured.
While walking through the sward make sure to record the number of readings taken by clicking the top counter. Sampling is done at random across the grazing area, so dung pats are eligible for measuring, and the only areas to avoid are poached and rutted areas.On slopes keep the plate meter vertical.
After walking through the sward, record the number of readings taken (C) and the closing reading (B) on the rising plate meter.
Use the following equation to calculate grass cover (KG DM/ha).
Closing reading (B) - Opening reading (A) x 158+330KG DM/ha
Number of readings (C).
To calculate the quantity of grass present in the whole paddock multiply the DM yield of each paddock by the area of the paddock in ha.
This should be repeated for all the paddocks that are intended for grazing. Do not measure paddocks that are likely to be cut as silage or grazed by other stock.
By adding all the individual paddocks that are to be grazed together, and dividing by the overall total area in ha, the average farm cover present within the grazing area can be calculated.