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Calculating fodder reserves on your land

By David Mason-Jones

Professional farmers and graziers should have the ability to make management decisions based on facts, not on guesses. One area where this is important is in calculating the amount of fodder reserves existing in your paddocks at any one time. Once this has been determined on a mathematical basis, decisions about the stocking rate and carrying capacity can then be based on logic rather than hunches.

To purchase new cattle or stock of any kind is expensive. It is a waste of money to purchase too many grazing animals because the country ‘looks good’ only to be forced to an early sale because there was not the amount of fodder in your paddocks as you first thought. Graziers are in the business of growing grass and then matching the stocking plan to the amount of grass available.

The skill of measuring fodder is therefore particularly important for new farmers or for farmers who have moved from a previous farm to a new location. The best time to carry out a fodder calculation is prior to purchasing a particular farm so that you can work out whether or not the farm really has the carrying capacity to support the stock numbers you intend to run.

In a Summer rainfall area another good time to measure fodder is in the weeks just before Winter sets in. This will reveal how much fodder is available for winter feed.

The steps to calculating fodder include:- Quarter square metre template. Make a simple but robust template which will measure out an area of a quarter square metre - 500mm x 500mm.

Random sampling site or sites. Drop the quarter square metre template randomly in the fodder growing in the paddock. Avoid the temptation to measure only the best stand of growth as this will not give a true reading for the whole paddock. Due to the fact that the density of grass growth may vary in an individual paddock, it may be best to repeat the random sampling process at several different sites in the paddock.

Cut the fodder within the template. Cut and gather the fodder growing within the template being careful not to cut the fodder to the bare ground level. If it is cattle that are intended to be run in the paddock, cut all fodder above 75mm height. If it is intended to graze sheep, cut all fodder above 35mm.

Weigh sample. Weigh the entire sample from the quarter square metre template area. This will give the total green weight of the sample including the water contained in the leaf material. At certain times of the year the grass growth may look abundant because it is green and lush but this can be deceptive when estimating total fodder reserves. The true food value of the fodder may be less than it looks because such a large part of the volume just consists of water.

Remove moisture. Once the total sample has been weighed, take a small portion of it, say 50gm, and dry it out so that the weight of the dry matter can be determined. The aim here is to find the ratio, or percentage, of dry matter in the sample compared with the total ‘wet’ weight. The food sustenance and nutrient value of the fodder is contained within the dry matter so this is the figure which is most important. The method of drying is to cut the sample into 25mm lengths to help the drying process and dry the sample in a microwave oven on high for five minutes. Caution must be exercised here so that the dry material does not catch on fire. One method is to place the cuttings around the edge of a plate and put a small bowl of water in the centre of the plate. This will keep some humidity in the microwave chamber and help prevent fire. NEVER leave the microwave during drying.

Weigh the dry material. After five minutes in the microwave, weigh the dry material. If necessary repeat the drying process until the weight of the sample stops diminishing.

Calculate the dry weight ratio. Compare the green weight of the microwave sample with the dry weight of the microwave sample and determine the ratio or percentage between the two. Using this ratio it is then possible to calculate the total dry weight contained in the original quarter square metre sample. Multiplying this by four gives the dry weight of fodder per square metre.

Extrapolate. To calculate the total dry weight of the paddock use the fact that there are 10,000 square metres in each hectare. If your paddock is 1ha, multiply the dry weight per square metre by 10,000 to give the weight of fodder in the paddock. If the paddock is 50ha multiply the 1ha result by 50.

Be realistic. Discount poor grass growing areas such as rocky ground, creeks and eroded areas, swampy ground, cattle camps and areas of weed infestation. To come up with the best estimate it is advisable to make three or four measurements in a paddock and come up with an average.

Using the result. The resulting figure can be expressed as the number of kilograms per hectare or by the gross number of kilograms in the entire grazing area. Compare this gross figure with the daily feed needs for the type of stock you are running. Once these figures are available it is a simple matter to calculate the appropriate stock holding capacity of the property by comparing the total fodder with the total stock and the likely time delay before new growth occurs.

When this article was written an experiment was conducted with a quarter of a square metre of Kikuyu. The green weight of he grass was 108gm (after allowing 6gm for the weight of the plastic bag). A sample of 50gm of the green grass was then dried to give a dry weight of 11gm. This means that the dry to green ratio is 10%. Therefore, applying the dry ratio to the total grass from the sample area, the total dry weight in the sample area is 11gm x 108/50 = 23.76gm. There-fore the dry weight per square metre is 23.76gm x 4 = .95kg. Extending this to a hectare this means that there is 9500kg per hectare of dry feed assuming that the cover of grass is even all over the land area.

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