Potassium and Phosphorus Fertilization of Grass Pastures

Pasture fertilization plays a vital role in the success of modern forage-based livestock production systems. Despite the vast scientific literature suggesting that forage crops can respond favorably to high levels of fertilization, the increasing costs of commercial fertilizers and environmental problems associated with improper pasture fertilization have prompted the need to re-examine optimum fertilizer rates, sources, and method of application that can sustain economic forage crop yields. While N typically represents the most limiting nutrient affecting forage production in the Southeastern USA, phosphorus and potassium are important components of sustainable forage production. Numerous studies have shown that highly productive warm-season grasses respond favorably to nitrogen (N) application (Haby et al., 1979; Burton and Hanna, 1995; Robinson, 1996; Silveira et al., 2007); however, potassium (K) and phosphorus (P) are often related to long-term persistence of grass pastures. It is important to emphasize that even in low-input systems where low yields are usually obtained, cultivated forage grasses require some level of fertilization. If nutrients such as P and K are not replaced, soil and forage productivity will gradually decline. The type of forage grown, soil type, and the type of production system (i.e., grazing pastures vs. hay fields) will determine which nutrients are needed and the proper application rate and frequency.
Numerous field observations in Florida suggested that importance of K and P fertilization of highly productive warm-season grasses such as Jiggs bermudagrass (Cynondon dactylon (L.) Pers.) and limpograss (Hemarthria altissima Stapf. and Hubbard) has been overlooked. There have been cases of bermudagrass decline reported in Florida as a result of deficiencies in K and P. These failures are typically observed in more intensive production systems such as hayfields where the vegetation is harvested up to five times a year during long periods. It is not uncommon to see forage fertilization practices focused solely on N fertilization. To further aggravate the issue, most soils used for forage production in the Southeastern USA exhibit poor fertility conditions and relatively low K and P concentrations. Under these conditions, comprehensive soil fertility programs that take into account crop nutrient requirements as a whole (i.e. K, P, and micronutrients) can greatly increase forage yields, nutritive value, and animal performance.
Another important aspect that supports a more holist soil fertility approach is that fact that low K and P supply in the soil may also limit the ability of the plants to utilize the applied N. While addition of N increases yield, it also stimulates additional uptake of other nutrients. If soil K and P supply is low, forage response to N fertilization can be also reduced. As the soil reserves became more depleted, crop responses to added N is expected to become less evident. In addition, because sandy soils often exhibit low cation exchange capacity and do not retain large amounts of K and P even after receiving fertilizer (Snyder, 1981). Snyder and Kretschmer (1986) suggested that the value of soil testing in P and K fertility programs for sandy soils may be limited. Rather, these authors indicated that plant tissue testing may be more useful than soil testing for predicting P and K deficiencies in grasses growing in sandy soils.


Justification

In the Southeastern US, forage-based livestock systems rely on warm-season perennial grasses such as bermudagrass and bahiagrass (Paspalum notatum Flugge). More specifically in Florida, bahiagrass is the predominant cultivated grass occupying approximately 2 million acres in the state (Newman et al., 2011). While bahiagrass is widely used in low input systems that require low (or no) fertilizer inputs, other grasses such as hybrid bermudagrass and limpograss are important forage crops for both dairy and beef cattle producers because of their greater yield potential and better nutritive value as compared to bahiagrass. Obviously, because of the greater yields, nutrient removal in harvested crops also increases.
Limpograss is a warm-season perennial grass used by Florida beef cattle and dairy producers because of its high quality, cool-season growth and tolerance of poorly drained soils. Currently limpograss represents the second most abundant cultivated forage crop in the state. Limpograss is also cultivated in Texas and Louisiana. This grass is well adapted to flatwoods soils and Florida’s climatic conditions and can be used for grazing and hay, silage and green chop production. Under good fertility and moisture conditions, limpograss produces 8 to 10 tons of hay per acre. Limpograss is one of the first grasses to initiate growth after a cool winter. In fall and early spring, this grass will produce more biomass than other warm-season grasses, especially in South Florida. Because of active growth in late fall, high digestibility, and the slower decline with increasing maturity compared to other grasses, limpograss is suitable for use as a stockpiled forage.
Jiggs is an improved hybrid bermudagrass that can be used for both hay and grazing. Jiggs is widely used in South Florida because it performs better than the other bermudagrass hybrids in poorly drained soils. However, it has less cold tolerance than Tifton 85 or Coastal. Jiggs is easy to plant by tops, does not have many rhizomes, and does not produce as much as Coastal or Tifton 85 in a drought. Quality is more or less equal to that of Coastal. Jiggs has fine stems and it has been used for hay, haylage, and grazing.
Both Jiggs bermudagrasses and limpograss require relatively higher fertilization levels as compared to other less productive grasses such as bahiagrass. Although these grasses are very responsive N, they also require higher K and P inputs to maintain adequate forage production. Snyder and Kretschmer (1986) demonstrated that dry matter yields of four warm-season grasses including limpograss and ‘Coastcross-I’ bermudagrass increased in response to K fertilization. Similarly, Adjei, M.B. et al. (2001) observed a linear increase in limpograss yields as P fertilization rates increased.
Fertilizer recommendations for Jiggs and limpograss hay production consist of 80 lb N, and 40 lb K₂O/a, and 20 lb P₂O₅/A (soil tested low or medium in P) after each cutting (Mylavarapu et al., 2009). For grazing, recommended N aplication rate is 160 lb N/A and up to 80 lb K₂O/A, and 40 lb P₂O₅/A depending on soil test results. The need for routine use of micronutrients has not been demonstrated.
Despite the University of Florida recommendations for K and P fertilization of Jiggs and limpograss hayfields and pastures, many forage production systems do not supply adequate K and P to replace that removed as harvested forage. Consequently, soil K (and P to a lesser extent) availability declines which may often result in poor stand persistence and greater incidence of diseases and insect damage. As soil reserves become more deficient in K and P, forage yield, nutritive value, and overall persistence will decline proportionately. Because these grasses use large quantities of K as compared to P, soil K deficiency often occurs first, particularly in highly-yielding crops cultivated in sandy soils with low K-buffering capacity. Numerous studies suggest that intensive management such as high N application, forage yield increase in response to K application is expected to be more pronounced (Robison, 1985).
The objective of this study is to evaluate Jiggs bermudagrass and limpograss responses to K and P fertilization. Emphasis will be placed on minimum fertilization regimens that can maintain optimum forage yield, nutritive value, and stand persistence. Because limpograss is often conserved as a stockpile grass, this study will also investigate the effect of harvest frequency (4- vs. 2-times per year) on limpograss responses to K and P fertilization.


Methodology

The study will be conducted on established Jiggs bermudagrass and limpograss fields at the University of Florida, Range Cattle Research and Education Center, Ona, FL. Soil is a Ona fine sand (sandy, siliceous, hyperthermic Typic Alaquods). Initial soil samples will be collected from the Ap horizon (to a depth of 15 cm) and analyzed for pH and Mehlich-1 extractable P, K, Mg, and Ca. Plot size will be 20 x 10 ft, with a 10-ft alley between plots. Treatments will be arranged in a split-plot design with three replicates. The experiment will be conducted for a minimum of 2 yr (2012 and 2013).
Jiggs treatments: the main plot will be N rate (80 or 160 lb N/A) and P (0, 20, and 40 lb P₂O₅/A) and K rates (0, 40, and 80 K₂O/A) will be considered subplots, for a total of 54 plots (2 N rates x 3 P rates x 3 K rates x 3 reps = 54).
The rates of 160 lb N/A, 40 lb P₂O₅/A, and 80 K₂O/A correspond to the University of Florida, Institute of Food and Agricultural Sciences (UF-IFAS) fertilizer recommendations for established bermudagrass. In this study, we will also investigate the effects of reduced N, P, and K rates (corresponding to 0 or 0.5--times the UF-IFAS fertilizer recommendations) on Jiggs bermudagrass responses. Nitrogen will be applied as ammonium nitrate and P and K as potassium phosphate and potassium chloride, respectively.
Forage will be harvested at 6-week intervals to determine dry matter yield and nutritive value. Dry matter yield will be determined by harvesting two 3- x 10-ft forage strips from each plot to a 3-inch stubble height using a forage harvester. The remaining herbage will be harvested to the same stubble height using a sickle bar mower and removed from the plots. Forage samples will be dried at 60^oC for 48 hr for dry matter yield determination. Oven-dried samples will be ground to pass through a 1-mm mesh screen in a Wiley mill (Model 4, Thomas Wiley, Laboratory Mill, Thomas Scientific, Swedebora, NJ) and analyzed for total N, P, and K concentrations. Crude protein will be calculated by multiplying N concentration by 6.25. Samples will also be analyzed for IVDOM using the two-stage technique described by Tilley and Terry (1963) modified by Moore and Mott (1974).
Limpograss treatments: Treatments will consist of a factorial combination of 2 harvest frequencies (6- and 12-week interval) and P (0, 20, and 40 lb P₂O₅/A) and K rates (0, 40, and 80 K₂O/A), for a total of 54 plots (2 harvest frequencies x 3 P rates x 3 K rates x 3 reps = 54). Nitrogen will be applied at a rate of 80 lb N/A.
Forage will be harvested to a 8-inch stubble height using a forage harvester. Forage samples will be processed and analyzed as described for the Jiggs study.
Soil Analysis: Prior and at the end of each growing season, soil samples will be collected from the 0- to 6-inches depth of each plot and analyzed for pH and Mehlich-1 P, K, Ca, and Mg concentrations
Statistical Analyses: Statistical analyses will be performed using Proc Mixed (SAS, 2001). Nitrogen or harvest frequency and P and K rates will be considered fixed effects, with replicates and their interaction considered as random effects. The PDIFF test of the LSMEANS procedure and single degree of freedom orthogonal contrasts will be used to compare means.