Determining the amount of forage you have

By Gary Sliworsky
OMAFRA Rep.

Following is part two of an article on determining the amount of forage you have available on pasture for your livestock.
Sample & Weigh - A more accurate method to calculate the amount of forage in a pasture is to harvest 2 or 3 square feet of grass, weigh the amount harvested and dry a representative sample to determine the amount of DM. (Note - 2 sq ft is 1’-5” × 1’-5”.) You should sample several areas of the pasture to get a representative value. The sample can be dried in a microwave oven.
Weigh a sample of approximately 100 grams and microwave for a couple of minutes. Re-weigh, microwave again for a minute and weigh again. Repeat this procedure until the weight does not change. Put a cup of water in the corner of the microwave to avoid damaging the microwave. Percent DM is determined by dividing the dry weight by the wet weight multiplied by 100.
For example, with a wet weight of 100 grams, and dry weight 20 grams, the DM will be 20% (20/100 × 100). If the weight from two square feet is 200 grams and the DM is 20%, then there is 20 grams of dry matter per square foot (200 × 0.2 ÷ 2). An acre (43,560 square feet) would have 871 kg of forage available for grazing (0.02 kg/square foot × 43,560 square feet). If we multiply this number by 2.2, the 871 kg is converted to 1,916 lbs.
Calculating Head / Acre / Day
If the grass you measured indicated that there was 1,916 lbs of DM available for grazing, then this would support 63,873 lbs of animal per acre (1,916 ÷ 0.03). For every 63,873 lbs of animal you need to offer one acre of pasture for each day that they are going to be in the pasture. Ideally this will be one day, and should not be more than 5 days because intake will drop. The best grass is eaten the first day, and by the fourth or fifth day the pasture is pretty well picked over. This is very important for stocker cattle where you are looking for good gains, because you want to have them eating the maximum amount of forage each day.
Stocker cattle that are moved to fresh pasture every one or two days throughout the grazing season should gain 300 lbs in a 5 month grazing season.
Knowing how much forage and dry matter that you are offering to your pasture animals will allow you to get maximum performance from both the livestock and the pastures.