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Using In-Season Response for Effective Management of Nitrogen Fertilizer in Bermudagrass Hay Pastures

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Bermudagrass is responsive to nitrogen fertilization particularly when the soil moisture is not limiting. Research has shown a linear relationship to added nitrogen in terms of yield and quality of bermudagrass. Plant recovery of applied nitrogen (N) fertilizer generally decreases with increasing rates in current production systems.

The general production practice is to uniformly apply most of the N early in the spring based on a yield goal, or the "yield per acre you hope to grow." In order to take advantage of the above-average growing conditions in dryland agriculture, it is better to set the yield goal above that of average yields. Yield goal clearly indicates the risk the farmer is taking when he calculates the amount of fertilizer for the crop before production. For every 1-ton yield goal of bermudagrass production, an average of 50 pounds of actual N fertilizer is required.

The presence of spatial variability in agricultural landscapes is an issue demanding careful consideration for efficient use of fertilizers. Research at the Noble Research Institute and Oklahoma State University by means of intensive soil sampling indicates that variability in soil properties occurs on a fine scale, as close together as square feet. Where major differences were observed, the variability in phosphorus and potassium was not as large when compared to nitrogen variability. Just as family members in the same house require different food to perform efficiently, crops' requirements also vary from one area to another within a field. This is probably due to the previously mentioned variability, as well as differences in potential plant use.

Different methods of variable rate technology (VRT) include the use of satellite imaging, grid sampling and high-resolution sensing by ground-based sensors. VRT improves nitrogen use efficiency, which is currently around 33 percent worldwide. This will increase the economic profitability to farmers, increase fertilizer-use efficiency and also decrease environmental risk associated with the use of crop nutrients. The Noble Research Institute currently is conducting VRT research using GreenSeeker® technology and calibration stamp technology.

The GreenSeeker sensor (Figure 1), a breakthrough technology recently developed by Oklahoma State University, uses red and near infrared wavelengths to predict the yield potential within the crop-growing period. It is different from typical precision agriculture because here every four square feet can be managed as an independent farm. Healthy plants absorb more red light because of absorption by chlorophyll. Healthy plants also reflect more of the near-infrared light. So, the amount of each type of light that's reflected back to the sensor provides a measure of crop health. This information is used to develop a normalized difference vegetative index (NDVI) value. Research at Lahoma, Okla., on wheat has shown that, even after 30 years, the check plot that didn't receive any nitrogen yielded 40 bushels of wheat. We are aware that an average of 2 lbs. N fertilizer is required for every bushel of wheat. Where did the crop get the nitrogen for 40 bushels of wheat? It came from mineralization of organic matter and deposition through rainfall, which will increase during warm, moist years.

To know whether there is going to be response for added N for a particular season, establish an N-rich strip (nitrogen non-limiting) in early April. An N-rich strip is an area adjacent to a pasture receiving at least 300 lbs. N/acre. No difference between the N-rich strip and a farmer's pasture suggests that sufficient mineralization had occurred and limited additional N was needed and vice versa.

Calibration stamp technology mounted on an ATV (Figure 2) applies a grid of nine 3-foot by 3-foot (Figure 3) plots with variable nitrogen rates of 0, 20, 40, 60, 80 and 100 lbs. actual N as UAN with computer-controlled spray nozzles. Even without this technology, the farmer can put four to five different rates of nitrogen in increments of 20 to 25 lbs. actual N in a field by hand on a small-scale basis preplant or at the time of breaking dormancy. N is applied midseason by looking at the response to N rates (i.e., if the 40 lb. rate and the 60 lb. rate look the same, then apply 40 lbs. and save money.)

This article is intended to introduce some exciting research taking place at the Noble Research Institute. More details will be given in subsequent articles, or contact Jeff Ball or Jagadeesh Mosali at (580) 223-5810 for more information.