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How Much Grass Have I Got?

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What a summer!!! It has been a real challenge this past growing season to balance the number of cows and grass growth. But what would life be without a good challenge. Maybe we need those challenges just so we can appreciate better weather and better cattle prices.

October is a good month to take a little stroll through the pastures and get a handle on how much grass there is in front of the cows. October marks the beginning of autumn when our warm season grasses quit growing, and the grass on hand (and the hay we cut during summer) is what the cows are going to winter on.

How do we determine how much grass? One small trick we use quite a lot is height/yield relationships. Each inch of forage, be it bermuda, an Old World bluestem, or native grass has a value in terms of pounds per acre. Each cow we own has a certain dry matter requirement. It is just a matter of putting the two together.

As an example, let's pretend that we have 100 acres of bermudagrass and 50 mama cows weighing 1,000 lbs each. As an average across the 100 acres the bermudagrass is seven inches tall, and each inch provides 230 lbs dry matter/ac.

    Total forage=7 inches x 230 lb DM/in/ac=1,610 lb DM/ac.
    1,610 lb DM/ac x 100 ac=161,000 lb DM.
    Cow Requirement for DM is 2.6% of body weight.
    1,000 lb/cow x 2.6%=26 lb/cow/day.
    26 lb/cow x 50 cows=1,300 lb DM/day for the herd.
    The 100 acres of bermudagrass will last these cows:
    161,000 lb DM/1,300 lb DM/day=124 days.


So, these cows have 124 days of forage in front of them assuming good rotational stocking and no more forage growth. But where did I get the 7 inches? To get the 7 inches I just used pad, and pencil; and of course I took the heights in my stroll through the pasture.

Where did I get the 2.6% of body weight for the cows? This is a pretty standard consumption figure for cattle. It can, and does, range from a low of 1% to a high of 4%. Consumption is determined by forage quality. The greater the forage quality, the greater the consumption due to higher digestibility; therefore, a faster rate of passage through the digestive tract.

Where did I get the 230 lb DM/inch? We have sampled many of the common forages in southern Oklahoma. Height/yield relationships have been developed for these forages given Noble Research Institute type management. A copy of this publication can be obtained by calling one of the forage specialists.

By knowing this kind of information, you can determine how many days of grass are ahead of your cows. It would also help you to determine if you have any shortfalls in your hay stores for this winter.