Published Mar 18, 2014

Grazing days calculations! Or, what the livestock coordinator does in her spare time.

By Margaret Chamas

The “grazing stick” is one of a number of pasture monitoring tools available to graziers.  It allows the grazier to roughly estimate the forage yield in a pasture or hayfield, based on average standing height of the forage.  This information is useful in determining the carrying capacity of the pasture: how many animals can be grazed on that pasture?  Or, given a set number of animals, how long can they graze?  Here are the how-to’s on estimating the amount of available forage, and a quick way to estimate the days of grazing animals will get off of it.

The use of the grazing stick is detailed in this University of Kentucky publication by Ray Smith, Mike Panciera, and Adam Probst.  While there are sticks sold that have the conversions and information printed on the sides, a yardstick or ruler is all that’s needed to start using this tool.  Measure the average forage height: set the base end of the stick or ruler on the ground.  Using your spread hand, flatten the forage next to the stick slightly, until you feel modest resistance.  Note the height of the flattened forage.  Take several height measurements throughout the pasture, and average the values.  Next, estimate the density of the stand.  How much bare soil is apparent when the forage is viewed from above?  Classify the pasture as low, medium, or high density – roughly corresponding to <75% density, 75-90% density, or >90% density.  Then take the average height and average density, and use the charts in the information sheet or on the stick itself to estimate forage yield.  Information about the class and weight of animals and grazing system can be used to further estimate grazing-days available.

Using the data from the Kentucky document, I’ve created a simple worksheet that will run some of the calculations for you.  Download it here: Simple Grazing Days Calculator

The “Yield vs height” tab presents a table listing the estimated forage yield at different forage sward heights.  Values are available for several different pasture types – select from the drop-down menu next to “pasture type” and choose the one closest to your pastures.  Then figure out which density is most appropriate.  Note that there is a range of yield values within each density.  If your pasture is between 75 and 90% density, for instance, but on the higher end – go for the “high estimate” column.  If you’re not sure, average them.  Find the average forage height you calculated on the lefthand side, and the intersection of the appropriate column and row gives you your approximate forage yield.

That will tell you roughly how many pounds of forage dry matter you have per acre.  The next tab – “Grazing days calculator” – helps to turn that into information more useful for planning.

The green cells here can be edited; the yellow are the results you get from your inputs.  Tables are given providing numerical values for animal types and grazing systems.  Identify the animal class that is appropriate (dry cows, milking cows, etc) and the number corresponding to it.  Enter that in the “Animal Information” section, along with the average animal weight and number of head.  The yellow cell tells you about how much forage those animals are expected to eat each day.

In the “Pasture Information” section, first input the estimated forage density.  This is similar to what was found on the last tab.  Within the “forage density” table, identify your pasture type, then whether the density is low, medium, or high; and whether you want to use the lower or upper range of that density.  Find the appropriate value and enter that in the cell.  Then identify the class number associated with the grazing system you’re using (continuous or rotational).  Put that in the appropriate cell, and fill in the forage height and pasture size.  The yellow cells now tell you how much of the available forage per acre the animals will eat, based on the grazing system involved.  Knowing this allows calculation of the total grazing days per acre that the pasture can sustain.  And the final cell is the total days of grazing from that pasture.

Of course, the most accurate way to do any of this is to observe the animals yourself and track how many days you get.  Many graziers can eyeball a pasture and will know roughly how long the animals can stay.  But exercises like this may help train your eye to better evaluate pasture carrying capacity.

The stick and all these calculations can be made more accurate by calibrating the stick yourself.  Clip forage in a 1-ft2 area to ground level, let the forage air-dry for a few days (or quicker yet, cut it into smaller pieces and microwave it a minute or so at a time until it’s dry but not burned).  Weigh the dry forage – in grams – and multiply by 100.  This gives an approximation of the lb of dry matter per acre.

More details, instructions, and guidelines are found on the U of KY grazing stick information sheet – be sure to read it!