Fertilize, overseed with warm-season legumes, or both?

May, 2013
by Joao Vendrmaini, UF/IFAS, Range Cattle REC, Ona

Warm-season perennial grasses are the main forage used for beef cattle production in Florida and although warm-season grass requires a minimum level of N fertilization to maintain sustainable biomass growth and nutritive value to cattle, forage-based cow-calf systems in Florida are characterized by extensive grazing with limited (or no) use of N fertilization due to the high cost of commercial N fertilizer. Favorable cattle prices in the last two years have allowed producers to invest in management practices to improve forage production and nutritive value with the objective to have a better calf crop. Nitrogen fertilization and overseeding warm-season legumes into warm-season grass pastures has been effective management practices to achieve the objectives described above.

Research was conducted in Ona, FL to compare forage production, nutritive value, and animal performance of heifers grazing bahiagrass pastures fertilized with 60 lb N/acre, or bahiagrass pastures overseeded with stylosanthes, or bahiagrass pastures that were not fertilized or overseeded (control). The fertilized pastures had additional 1,000 lb DM/acre/month when compared to overseeded pastures and control. This additional forage production would allow greater stocking rates, which would be the equivalent of two 400 lb heifers/acre. The fertilized and overseeded pastures had similar CP concentration (12%) and were greater than the control pastures (10%). There was no difference in average daily gain of the heifers among treatments. In general, overseeding legumes or N fertilization may result in improvement in average daily gain when the crude protein in the warm-season grass is limiting. A study conducted in Gainesville, FL demonstrated that steers gained additional 0.6 lb/d when grazing limpograss pastures overseeded with aeschynomene when compared to steers grazing limpograss pastures fertilized with nitrogen. The additional nitrogen provided by the stylosanthes resulted in added 45 lb N/acre in the biomass of overseeded pastures when compared to pastures that were not overseeded. Considering that there are several variables involved in nutrient cycling from litter and excreta, such as heterogeneous distribution of excreta and slow decomposition and mineralization of litter, which would make the additional N present in the overseeded pastures not accurately comparable to the same levels of N fertilization, this greater N concentration and potential greater litter decomposition rates of legumes may contribute to greater N supply to the bahiagrass and may result in greater forage production in the subsequent years.

The main benefit of the nitrogen fertilization is the immediate increase in forage production, however, it has limited or no residual effect. If the producer is annually fertilizing the pastures with similar fertilization levels and management, the nitrogen fertilization will probably maintain the existing productivity of the cow-calf operation. On the other hand, if the producer fertilize areas that have not been fertilized previously or increase the nitrogen fertilization levels, it is necessary to be prepared to harvest the excess forage and/or increase stocking rates.

On the overseeding standpoint, establishment is a crucial factor that would dictate the effectiveness of the warm-season legumes. The two most important factors are 1) suppressing the warm-season grass to decrease the competition with the legume at establishment (grazing short, burning, disking, etc) and 2) adequate soil moisture conditions. It is recommended to overseed warm-season legumes into warm-season grass pastures in South Florida in early March when temperature and daylenght are not climatic limiting factors. Once the legume is established, fall management is important for seed production and potential reseeding of the legume in the subsequent years. Conversely, under the current scenario of high N fertilizer cost, producers may choose have an extended grazing season and use the legume as an annual crop and replant every year. As mentioned above, the benefits of nitrogen added to the pasture by the legume may be observed gradually over the years due to the decomposition and distribution of the plant litter and excreta.

Overseeding areas with greater soil moisture with warm-season grass species with limited nutritive value and fertilizing pastures with forage species with better response to nitrogen fertilizer (i.e. bermudagrass and stargrass) may be an effective and complementary use of these management practices.  The choice of fertilizing and/or overseeding must be dependent upon the objectives of the production system and economic feasibility of these management practices.
For questions related to warm-season legumes and pasture fertilization, please contact Joe Vendramini jv@ufl.edu.

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