The Grass Wedge – taking the guess work out of grazing management!

While every dairy farmer should aim to provide his herd with excellent quality grass throughout the grazing season, this is not always easy to achieve.  One of the main difficulties that many farmers face is to identify if the quantity of grass that they have available for grazing is on target to meet their herds’ requirements. To help overcome this difficulty, the ‘Grass Wedge’ has been developed to clearly demonstrate grass supply across the grazing area. A grass wedge graph will feature each week within the GrassCheck bulletins during 2010, and the aim of this article is to explain the basics of what a grass wedge is, and what it shows.

The grass wedge combines a number of measurements from a grazing system, and allows the balance between a farms grass supply and the herd’s grass demand to be determined. A grass wedge has two distinct parts:

1. The vertical bars – these represent the grass cover within each grazing field.

2. The diagonal target line – this is a straight line drawn between the target pre-grazing grass cover and the target post-grazing grass cover. The pre-grazing target cover can be calculated from the herds grazing stocking rate, grass intake, grazing rotation length and post-grazing grass cover (residual). During the main grazing season pre-grazing targets are generally between 3,000 and 3,300 KG DM per ha, with post-grazing targets between 1,600 and 1,800 KG DM per ha.

The starting point to produce a grass wedge for your farm is to walk the grazing platform and measure the quantity of grass within each field intended for grazing. This can be achieved using a rising plate meter or by visually assessing the quantity of grass present against the height of a wellington boot. Individual grass covers for each field on the farm are then ranked from ‘highest to lowest’ to produce the grass wedge bar chart. This is most easily done using a computer software package.  AFBI and CAFRE will be developing a grass wedge programme during the coming months, and this will be available on the internet.

The grass wedge illustrates how the grass available on the grazing fields (bars) compares with herd demand (target line). The key benefit of a grass wedge is that it can be used to identify current and future grass supply issues. In the grass wedge shown in Figure 1 there are 12 bars representing 12 fields. This wedge shows that grass supply is on target with herd requirements (the tops of the bars are close to the target line). The fields that should be grazed next (red bars) are close to the pre-grazing target of 3,100 KG DM per ha, while the fields that have just been grazed (blue bars) have been grazed down to the post-grazing target of 1,600 KG DM per ha. The remainder of the fields (grey bars) are also close to the line, indicating that grass supply across the whole grazing area is on target to meet current herd demand. However, a change in growth rate could change this balance, and therefore it is important that a grass wedge be produced regularly throughout the grazing season.

A second grass wedge is shown in Figure 2. In this example grass supply is not on target. Although the fields that are about to be grazed (red bars) and those that have just been grazed (blue bars) are on target, there is a grass shortage on the fields that the cows are due to graze in 8-10 days (the yellow bars that are below the line). Having this ‘early warning’ of a future grass shortage (or surplus) can provide the confidence required to make the correct management decisions in a timely manner. For example, if the wedge highlights a future grass shortage then additional fields could be added to the grazing rotation or supplementary feed levels increased. However if the wedge highlights a future grass surplus, then fields can be immediately cut and removed from the grazing rotation.

A grass wedge can be a very powerful grassland management tool. While it takes time to collect the information necessary to produce a wedge, setting aside time for a weekly farm walk should be a priority for all grassland farmers. Transforming this ‘field information’ into a grass wedge makes it very easy to compare actual grass supply with the herds’ grass requirements. This allows timely and confident grassland management decisions to be made. For further information on how to calculate a grass wedge contact Andrew or Martin, or alternatively consult your local CAFRE Dairy Development Adviser.