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Should Nitrogen be applied on Bermudagrass and Old World Bluestem this month?

Posted Aug. 1, 1997

Our fertilizer recommendations often include a statement suggesting you apply nitrogen on some portion of your total introduced grass acreage in August. A related statement also says that nitrogen rate, timing, and acres to treat depends on forage supply and expected need. In early August you should evaluate your existing forage supply and the amount of forage needed from then until next spring. If additional forage is needed, consider applying nitrogen to increase the standing forage or purchase hay. What is the best option?

In the April issue of this newsletter we presented information showing 85 to 90% of the season total bermudagrass was produced by August 15 if no fertilizer is applied or if the last fertilizer application is in June. On most upland sites in southern Oklahoma we expect a total yield of about 2500 pounds of dry grass per acre with no fertilizer and about 6000 pounds when 150 pounds of actual nitrogen per acre is applied before July. This would indicate a potential yield of 250 to 375 pounds of grass per acre after August 15 with no fertilizer and 600 to 900 pounds with spring fertilizer applications.

Bermudagrass forage yield information from limited one and two year studies indicate that August nitrogen applications of 50 to 75 pounds of actual nitrogen per acre will increase forage yields at the rate of 12 to 15 pounds of forage per pound of applied nitrogen. If your bermuda pasture would yield 600 pounds of forage per acre from August 15 until frost, applying 200 pounds of 34-0-0 per acre (68 pounds of actual nitrogen) in August would potentially increase the yield to 1416-1620 pounds per acre.

Forage clippings in early October show the crude protein content of bermudagrass (if it was grazed or mowed in August) can be 8-10% in unfertilized sites and 12-15% in areas fertilized in August. During the winter of 1996-97 a bermuda paddock at our Headquarters Farm was clipped every month. Bermuda that was 8.9% crude protein and 59% TDN November 26 contained 6.5% protein and 57.6% TDN January 24. This is only one year's observation so rainfall amounts and timing could influence the rate of change in forage quality.

How can you use this information to help you decide whether or not nitrogen should be applied on your grass this month? If you will need additional forage this fall, use the expected responses to calculate the cost of increased yields.

Then compare those costs to buying hay, renting land, or reducing livestock numbers. If nitrogen costs $0.28 per pound of actual nitrogen, the 68 pounds mentioned above would cost $19.04 and the per ton cost of 1620 pounds of forage would be $23.51. You may also want to give some credit to increased protein values. A WORD OF CAUTION The potential responses listed above are averages and little response will occur if rainfall is very limited in August and September.