published in


February 2013

Fertilizing with nitrogen or overseeding with warm-season legumes?

by Joe Vendramini

Ona Report - Dr. Joao Vendramini

For questions or comments regarding this publication contact: Dr. Joao Vendramini, University of Florida, IFAS

Warm-season perennial grasses are the main forage used for beef cattle production in Florida. Although they require relatively lower soil fertility conditions than cool-season forages, minimum N inputs are required by warm-season grasses to maintain sustainable forage production and nutritive value to cattle. However, because of high cost, the use of commercial N fertilizer in Florida pastures has been limited.  This scenario has slightly changed during the past two years in response to better cattle prices. More favorable economics have allowed producers to invest in management practices that can increase forage production and nutritive value and, consequently, improve calf crop. Among these management practices, nitrogen fertilization and the use of warm-season legumes overseeded into warm-season grass pastures have become attractive options to improve forage and animal productivity. 

One of the main benefits associated with commercial nitrogen fertilizer application is the immediate increase in forage production; however, limited residual effects are normally observed in subsequent year following fertilization. This means that annual application of commercial fertilizer often is necessary to maintain the same level of forage and animal performance. When greater levels of nitrogen fertilization are used or areas that have not been fertilized in previous years are fertilized, it is important to be prepared to harvest the excess forage and/or increase stocking rates. Several factors may also affect forage production such as rainfall, diseases and pest, thus forage response to nitrogen application can be variable depending on the year. 

picture of Dr. Joe Vendramini working in a fieldResearch conducted at the Range Cattle Research and Education Center in Ona, FL  compared forage production, nutritive value, and animal performance of heifers grazing bahiagrass pastures either fertilized with ammonium nitrate at a rate of 60 lb N/acre, overseeded with stylosanthes, or bahiagrass pastures receiving no fertilization or legume (control). Pastures fertilized with ammonium nitrate produced 1,000 lb DM/acre/month more forage than bahiagrass pastures overseeded with legume or receiving no fertilizer/legume (control). This additional forage production would allow greater stocking rates equivalent to two 400 lb heifers/acre. Bahiagrass pastures fertilized with nitrogen or overseeded with legume showed similar crude protein concentration (12%). Control pastures (no N fertilizer or legume) showed lower crude protein concentration (10%) than the other treatments. There was no difference in average daily gain of the heifers among the three treatments. In general, overseeding legumes or nitrogen fertilization can improve average daily gain when crude protein in the warm-season grass is limiting. A study conducted in Gainesville, FL demonstrated that steers gained an additional 0.6 lb/d when grazing limpograss pastures overseeded with aeschynomene as compared limpograss pastures fertilized with nitrogen fertilizer. 

Overseeding stylosanthes into bahiagrass pastures resulted in approximately 45 lb N/acre more in the above ground biomass than bahiagrass alone. Because several factors affect nitrogen cycling in grazed pastures, a direct comparison between the amount of nitrogen added by the legume in the overseeded pastures and similar level of nitrogen fertilization cannot be made; however, the additional nitrogen provided by the legume can improve nitrogen supply and, consequently, increase forage production and quality in the subsequent years. 

Establishment is a crucial factor that may impact the effectiveness of warm-season legumes in providing nitrogen to the grass. One of the most important factors for successful establishment of overseeded legume is to suppress the growth of the warm-season grass so the legume can better compete during the establishment period. This can be accomplished by grazing the grass short, burning, disking, or using herbicide. Another important aspect to consider is adequate soil moisture conditions. In general, the recommended period to overseed warm-season legumes into warm-season grass pastures in South Florida is early March when temperature and daylength are optimum for the legume; however, the soil moisture is not always ideal. Once the legume is established, fall management is important for seed production and potential reseeding of the legume in the subsequent years. Another option that may be attractive particularly when nitrogen fertilizer prices are high is to extend the grazing season by using the legume as an annual crop and re-establish the crop every year. As indicated previously, the benefits of the nitrogen added to the pasture by the legume will not be as immediate as commercial fertilizer. The legume-nitrogen only becomes available to the forage grass as it is recycled back to the system as plant litter and animal excreta.

The choice of fertilizing and/or overseeding legumes must be dependent upon the objectives of the production system and economic feasibility of these management practices. For questions about this article, please contact Joe Vendramini at jv@ufl.edu.