What is a Pasture Inventory and How Do You Do It?

A pasture inventory provides an assessment of what you have available in your pastures at a given point in time. Just like a retail sales business conducts periodic inventories to know what they have on the shelf to sell, any grazing farm or ranch should know what they have available in the pastures as standing feed. We generally try to conduct an inventory every two weeks on irrigated or high natural rainfall pastures and monthly on rangeland.

The basic process is to look at every pasture and come up with a measure or estimate of available feed in that pasture. In my mind there are three ways of doing the inventory. Each method evolved out of the previous approach. I will briefly outline each method.

The first approach is to actually try to measure the dry matter yield of forage per acre through either clipping forage or some other non-destructive method such as grazing stick, rising plate, or one of the electronic or digital tools. While many grazing researchers believe this to be the most accurate method, I would argue that it is not the most meaningful. The process is also cumbersome and contains many potential sources of error. This inventory approach gives us a dry matter yield estimate for each pasture in our grazing cell. To actually come up with an expected livestock carrying capacity, we then apply estimates of daily intake and utilization targets. The math often becomes cumbersome.

The example would be our assessment says there is 2800 lbs/acre of standing forage. We choose a utilization target of the standing forage. If we choose 50%, then the amount of forage we plan to remove is 1400 lbs/acre. We estimate our cows to weigh 1200 lbs and they have a daily intake rate of 2.8% of their body weight. That would put the needed forage for each cow at 33.6 lbs/cow/day. We divide the daily individual requirement into the amount of forage to be removed from each acre (1400 lbs/acre ÷33.6 lbs/cow/day) and we find we could put 41.7cows on one acre for one day. We have four sources of possible error in our process: estimates of available forage, degree of utilization, cow weight, and projected intake. This is why so many research projects run by the numbers turn out so poorly.

The second method is based on measuring the height of the pasture and realizing that an inch of pasture can provide a predictable amount number of grazable forage. We can bypass measuring the actual pounds per acre of available forage and jump right to a AUD/acre yield per inch of forage grazed. We typically use the values of 5 animal unit-days/inch of grazable forage for fair pastures, 10 AUD/inch for good pastures, and 15 AUD/inch for excellent pastures. With a little experience, you learn to adjust the value a little one way or the other based on specific conditions. This approach still involves measuring the height of the forage, choosing a target residual height and doing a calculation.

In this process we go out and measure the pre-graze pasture height. Let’s say we come up with 14″. We decide we want to leave 6″ which means we plan to remove 8″ of available forage. If we called this a ‘good’ pasture, we would be hoping to remove 80 AUD/acre. Sources of error are was that pasture really 14″ tall, what size are our livestock, and did we really leave it at 6″ residual height.

On the day you forget to take your yardstick, you might come to the realization that you have a pretty good idea of what 80 AUD/acre pasture looks like based on past experience with your grazing stick. You simply look at the pasture and make an AUD/acre estimate, count your animals, and set up your temporary fence based on dividing the number of animals in the herd by your AUD/acre estimate. Yes, you need to adjust your number of head to number of AU equivalents.

Come back tomorrow and assess the situation. If the pasture is grazed shorter than your target, you guessed wrong. Adjust the AUD/acre number downward. If it is taller than you expected, adjust the number upward. Repeat this process about for about 30 daily moves and your eye will statistically be as accurate as any tool we have available for measuring forage yield.

This visual estimate of stock days/acre is the method I use. It does not require me to carry any tools with me other than a pasture map and pen to record my estimates. A more tech savvy person might do it all on their smart phone. Here is an example of a pasture inventory map. I like using the map because it gives me better spatial reference to where I am in the process.

The white number is the pasture ID number and the black number is the observed inventory value as AUD/acre. An important note is the estimate is based on what you would be willing to remove from the pasture if you needed to. We typically would not want to graze a pasture with anything less than 30 AUD/acre available. My optimum range for grazing is between 60-90 AUD/acre. A zero is not bare ground. It is a paddock grazed to the appropriate residual for that time of year. If I graze too severely, I will assign a negative value based on how many AUDs I removed beyond the appropriate level.

This is very much a learn-by-doing process, but once you have mastered the estimation part of the inventory, it becomes a valuable tool for planning your grazing for the next few weeks to few months depending on the environment. In the next issue of On Pasture we will explore taking the inventory data and turning it into useful management information.