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Pasture & Range: July 2000 Soils: July 2000

by Wadell Altom

Producers and those who do research and advise producers often talk about the influence of inputs and management on crop and forage yields. Recently, one of my co-workers commented that he had never seen any data showing the influence of rainfall on yields. Because I was struggling for a topic for this article, I used some research done from 1972-75.

Rainfall
Year April-Sept (In.) Total (In.)
1972 12.6 25.1
1973 31.2 54.4
1974 33.5 49.6
1975 32.6 48.8
This fertilization study was conducted on 'Midland' bermudagrass grown on a Chickasha loam soil at the Noble Foundation's Pasture Demonstration Farm. The study was designed to evaluate nitrogen, phosphorus, and potassium effects on 'Midland' bermudagrass hay yields. Selected treatments are listed below.

Forage yields in 1972 were extremely low, but so was rainfall (average rainfall about 36 inches per year). In 1973, both were above average. Although total rainfall was slightly lower in 1974 than 1973, the April-September rainfall total was above average, and yields were higher with the lower fertilizer rate and when no fertilizer was applied. In 1975 total rainfall was almost the same as in 1974, and yields were also similar.

Effects of Fertilizer on
'Midland' Bermudagrass Forage Yield Treatment Yield
  (Lbs. Oven-Dry Forage/Ac.)  
(Lbs. N-P2O5-K2O/Ac.) 1972 1973 1974 1975 Average
0-0-0   772 2,061 3,807 3,072 2,428
75-40-60 1,493 4,165 5,051 4,667 3,844
150-40-60* 1,944 6,833 6,876 7,073 5,682
*Nitrogen split into two applications (75 pounds in April and 75 in June).

These data show a definite relationship between rainfall and bermudagrass forage production and suggest that rainfall received during the growing season is critical. Nitrogen rate and time of application may interact with rainfall to influence forage yield, since the 150-40-60 fertilizer treatment (with 75 pounds of nitrogen applied in April and 75 in June) yielded more uniform crops in 1973, 1974, and 1975 than the 75-40-60 or 0-0-0 treatments.

A complete discussion of the influence of proper fertilization on water use efficiency would require another article. Information in an Oklahoma State University Extension newsletter quotes research showing that, without proper fertilization, 18 to 20 inches of water is required per ton of bermudagrass forage, while only 3.5 inches per ton is required when fertilization is proper.

If you can predict the weather, we can help you plan the most practical fertilizer application rate and schedule.
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