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Calculating Forage Demand and Forage Availability

By Jace Stott
A common conversation I have with producers usually goes something like this, “I’m renting out 50 acres to my neighbor who wants to run 30 cows on it, does that sound about right to you?” This is a perfectly legitimate question, however, more details are needed on both the cattle and the pasture to fully answer this question. 
Determining appropriate stocking rates can seem daunting to some producers because, like me, they probably have less than fond feelings towards mathematical calculations. However, determining appropriate stocking rates does not have to be complicated. The key is remembering two main elements required for stocking calculations, Forage Demand and Forage Availability. 
When calculating stocking rates, keep the big picture or main elements in mind, those being FORAGE DEMAND and FORAGE AVAILABILITY. This is intuitive, even in a simplistic stocking question like the example given above, the producer recognizes that there has to be some sense of demand (30 cows) and availability (50 acres). All subsequent information required to calculate appropriate stocking will fall under the categories of FORAGE DEMAND or FORAGE AVAILABILITY. Though many aspects of stocking can be calculated (following link is tutorial for stocking calculations), many producers just want a basic idea if they will have enough feed for the number of cattle they are grazing. The purpose of this article is to provide an example on how to do a quick and basic comparison of forage demand and forage availability. 
Calculating forage demand captures three important elements animal size, length of time grazing, and number of animals. Animal size is taken into account using an Animal Unit Equivalent (AUE). The AUE is an estimate of the amount of forage an average cow will eat. The baseline for the AUE system is a 1000 pound (lb) animal, in other words, a 1000 lb animal equals 1.0 AUE which will eat on average 680 lb of forage (Dry Matter) or 780 lb of forage (Air Dry). Each hundred pounds over or under the 1000 lb baseline is either an addition or subtraction of 0.1 (e.g. 1100 lb cow equals 1.1 AUE). To combine AUE with amount of time grazing and number of animals grazing one simply needs to multiply. For example, if 30 cows weighing 1200 lb were grazing for 6 months the forage demand equation to calculate Animal Unit Months (AUM) demand would look as follows: 1.2 AUE*30 head*6 months = 216 AUM’s. This is our demand.
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