THE FLORIDA CATTLEMAN AND LIVESTOCK JOURNAL
The importance of micronutrient fertilization of warm-season grass pastures in Florida
Micronutrients are defined as essential plant nutrients that are found at concentrations less than 0.01% in the plant tissue. There are eight micronutrients identified as essential to plants, boron, chlorine, copper, iron, manganese, molybdenum, zinc, and nickel. The average micronutrient concentration in warm-season grass plant tissue collected from different regions in Florida is presented in Table 1. Although these values were generated from different locations, soil types, forage species, and management regimes, they do not represent deficiency or sufficiency and fertilization with the specific micronutrient does not necessarily result in an increase in forage production or nutritive value. Despite the vast literature documenting average micronutrient concentrations in warm-season forage species, information regarding the critical tissue levels and the effects of micronutrient application on forage production and nutritive value are still limited.
A number of studies conducted in central and south Florida concluded that micronutrient fertilization did not increase herbage production on grazed bahiagrass, limpograss, and bermudagrass pastures. Although micronutrient tissue concentrations typically increase with nutrient application, several other factors such as nutrient cycling through animal excreta, soil type, nitrogen fertilization, and soil pH affect plant response to micronutrient fertilization. Approximately 85% of the nutrients consumed by cattle returns to the pasture via excreta deposition; however, micronutrient return through excreta is not uniform, causing significant variability in nutrient concentration in the soil. In addition, soil type, moisture conditions, microbial activity, and fertilization program also affect micronutrient availability in the soil and subsequent plant response to added micronutrient fertilizers. Unlike grazed pastures, in hayfields where limited nutrient cycling occurs, complete fertilization with macro and micronutrients may promote forage growth and sward persistence.
Soil pH is an important factor that can affect pasture productivity, persistence, and fertilizer use efficiency. Soil pH controls the availability and uptake of soil nutrients; therefore, adjusting soil pH is the first and most important step when addressing soil fertility programs for forage production. If the soil pH is not adequate for the forage, it should be adjusted before pasture fertilization. Different forage species have different pH requirements so soil pH should be managed according to the soil and forage conditions. For instance, bahiagrass is more tolerant to low pH than most warm-season grasses; however, it is also very sensitive to high pH (> 6.5). In a greenhouse study conducted in Gainesville, the authors observed that bahiagrass had decreased production and persistence when it was growing at the pH range of 6.1 to 6.5. On the other hand, it is perceived that bermudagrass is less tolerant to low pH and more tolerant to high pH than bahiagrass.
Several field observations in Florida suggest that bahiagrass turns yellow in the spring and this symptom is routinely associated with micronutrient deficiency, more specifically iron deficiency. A grazing study conducted in Gainesville to test the effects of foliar application of micronutrients on production and nutritive value of bahiagrass pastures found that foliar application of micronutrients changed the color of the forage but did not increase production or nutritive value. Bahiagrass pastures in this study showed adequate pH, and fertility and grazing management was conducted properly, thus the main variable affecting plant responses was the micronutrient additions. Although it is widely known that excessively high soil pH (> 6.5) may detrimentally affect bahiagrass growth and plants are typically yellow; no evidence has been found that suggest that application of micronutrients increases forage production under this condition.
Soil testing alone is a poor indicator of micronutrient availability in the soil. Although limited data is available on critical micronutrient tissue concentrations, tissue testing is suggested to be a good tool to determine micronutrient deficiency in the plant. The concentration of micronutrients in plants can vary depending upon the plant maturity, season of year, and nitrogen fertilization, among others factors.
In summary, micronutrients are essential for forage production but micronutrient application to grazed pastures may have limited impact on pasture performance because the amount of micronutrients either present in the soil or recycled through animal excreta may be sufficient to meet the crop requirements. Addressing soil pH and macronutrient availability are certainly the most important steps for pasture soil fertility programs. Positive effects of micronutrient fertilization are likely to occur under mechanical harvest forage production systems (i.e. hay, silage, sod) because of the limited nutrient cycling and removal of nutrients with the harvested plant biomass.
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Table 1. Forage micronutrient concentration in different regions in Florida (adapted from Mcdowell et al. (1982)
|Micronutrient||Dry matter concentration (ppm or mg kg-1|