Nitrogen (N) is the most important plant nutrient, and, when deficient, it significantly reduces productivity. Primarily, nitrogen improves plant efficiency, resulting in increased forage nutritive value and reduced fiber content. However, the overall effect is dependent upon environmental conditions like soil moisture and application schedule. Application of N once plants have established sufficient root systems is meant to improve N capture and prevent leaching. In environments experiencing high precipitation, there is a theory that N use efficiency may be improved through a split N application schedule. The hypothesis is that the first N application increases shoot growth and proliferation of roots with minimum N loss, while a second application at the peak of vegetative growth enables the plant to optimize nutrient uptake, growth and harvest index.
In bermudagrass grown for hay and expected to yield multiple cuts, a single N application results in improved growth, increased biomass and better first harvest forage quality. With the single application strategy, both the yield and quality of subsequent harvests may be compromised if soil N is depleted prior to regrowth. In high precipitation areas, because the potential exists for significant N loss, split application strategies may be a viable alternative for increasing forage quality. However, research conducted by the Noble Research Institute in south-central Oklahoma revealed that this assumption may not always be correct (Figure 1).
Nitrogen fertilizer applied in a single dose in spring gave better quality forage in spring and summer compared to split applications that included a summer treatment. In semi-arid environments like south-central Oklahoma, the relatively wet spring allows the plant to take up sufficient N and increase vegetative growth, crude protein and total digestible nutrients. After spring harvest, underground plant tissues seem to contain substantial amounts of N and N -containing compounds that are utilized for summer regrowth. However, under split N application, the spring-applied N seems to be used for spring vegetative growth, leaving very little available for underground tissue storage. The second N application expected to increase summer forage quality may not be readily accessible to the plant because of stress related to summer environmental conditions. The low response to the second application of N is most likely related to plant stress caused by the region's hot and dry summers. This lack of sufficient moisture exacerbates stress in the plants resulting in low N absorption and decreased forage yield and quality.
Therefore, if south-central Oklahoma maintains its historical moisture accumulation levels, it is preferable to apply all necessary N in the spring. This will allow the plant to capture and absorb N under the relatively good spring conditions. The absorbed N incorporated into the underground tissue in spring becomes available to the plant to sustain growth throughout the summer.