This topic is perhaps one of the most misunderstood in pasture and forage fertility issues. It is best to define a few terms before we make too many statements. Forage Quality and Soil Fertility
The best measure of forage quality is animal/livestock performance. Since this is only directly measured with large scale, expensive, feeding trials, we have to have indicators of forage quality that are relatively easy to measure. The old standards are crude protein (CP) which we use to estimate the protein feeding value of a feedstuff, and total digestible nutrients (TDN) which provides an estimate of digestibility. For the most part few labs determine TDN any longer. Rather, they run acid detergent fiber (ADF) and use it to predict TDN. We should be using the ADF directly rather using a predictive value but general public acceptance of ADF has been somewhat slow. A more in-depth treatment of forage quality is available on the web at: Forage Quality. Forage maturity has, by far, the greatest affect on CP and TDN levels in forages.
Soil potassium level/fertility does not affect crude protein and digestibility (TDN, ADF) of forages. Deficient or low potassium fertility levels will most assuredly reduce forage growth, e.g., can become first limiting nutrient and decrease overall yields. Potassium is assimilated in luxury amounts by most forage species. Forages, typically, will accumulate two to 20 times sufficient/required levels of potassium when it is available in the soil.
Soil phosphorus level/fertility also does not affect crude protein and digestibility (TDN, ADF) of forages. Like potassium, low soil phosphorus can be growth limiting. Phosphorus is typically not consumed in luxury amounts like potassium and will generally show on forage analysis as 0.2 to 0.3% composition on a dry matter basis. Phosphorus may be low enough in forage plant tissue that it becomes deficient in the grazing livestock diet. When phosphorus is this low in the soil, plant growth will most definitely be reduced/limited. In terms of animal nutrition phosphorus should be fed to the livestock in a mineral supplement to correct the deficiency. In the long run however, the soil phosphorus levels must be adjusted for adequate forage growth.
Soil nitrogen level/fertility does not (directly) affect digestibility but does directly affect forage crude protein levels in grasses, with much less effect on crude protein levels in legumes. Within reason, the greater the nitrogen fertilizer applied the higher the forage grass crude protein; there is, of course, an upper limit to this affect. Nitrogen fertility levels should be based on realistic yield expectations however, and forage crude protein levels should be managed by plant maturity at harvest, whether by haying or grazing management.
From a practical field perspective the fertility levels of the macro and micro-elements do not affect crude protein and digestibility either. But once again, if they become deficient they can directly affect yield. In perennial pastures and hay fields the major economic impact of inadequate fertility level is probably not yield but the persistence/longevity of the stand.
There are other aspects of forage quality that can be directly affected by soil fertility and fertilizer application. We typically refer to these factors as anti-quality components. The two most common are nitrate accumulation and the tall fescue toxicosis problem.
Nitrate Toxicity There is no question that nitrogen application to pastures improves production and crude protein content of forages. However, moderate to high nitrogen fertilizer rates can elevate the forage nitrate concentrations to toxic levels. This is especially true if the plant is a nitrate accumulator or has been environmentally stressed.
Nitrate toxicity is a usually a problem because some plants are "luxury" nitrogen consumers. This means that when high levels of nitrogen are available in the soil, plants will take up more than is immediately needed. This "luxury" consumption can result in a buildup of nitrate in the plant (especially in the lower stem). Toxic nitrate levels are often observed 1-4 weeks following high rates of nitrogen fertilizer. Sorghum, sudangrass, soghum-sudangrass hybrids, millets, and corn are all prone to accumulating high amounts of nitrates, especially when fertilization occurs in conjunction with plant stressors like droughty, cloudy or cool weather. Although nitrate toxicity is usually associated with summer annual forages; even tall fescue, cereal grains and bermudagrass can contain toxic nitrate levels when grown under heavy nitrogen fertilization (Figure 1; Hojjati et al. 1972). Nitrate levels generally peak in plants 1-3 weeks after fertilizer application and gradually decline over time; however, plant nitrate concentrations can exceed toxic levels for several weeks after they peak depending on nitrogen application rate and plant growth conditions. Several field test kits are available through state extension personnel. When in doubt it is wise to sample forages before grazing, cutting or feeding. More information is available at: Nitrate Toxicities on the web.
Fescue Toxicosis Animals grazing tall fescue pastures often have lower forage intakes, decreased animal production, rough hair coats and higher body temperatures. A fungus contained inside tall fescue was identified in the 1970's by USDA researchers. This fungus produces toxic alkaloids which appear to cause the symptoms listed above. Several studies have examined the effects of soil fertility on alkaloid production in tall fescue. The results are summarized below.
Nitrogen Initial observations of cattle grazing tall fescue indicated that fescue toxicosis was most severe on pastures receiving heavy applications of poultry litter. This led researchers to believe that high nitrogen rates may increase alkaloid production, which increases the severity of tall fescue toxicosis. The effects of nitrogen fertilization on toxic alkaloid production in tall fescue are relatively unclear, but overall results indicate that nitrogen application does increase toxic alkaloid production in tall fescue. Several research studies have shown that toxic alkaloid levels increase with nitrogen fertilization (Figure 2, Rottighaus et al. 1991). Field observations of cattle grazing heavily fertilized tall fescue pastures also suggest that high nitrogen application rates increase toxicity of tall fescue. However, other studies have reported no alkaloid response to nitrogen fertilization. Inconsistent results may be due to form of nitrogen fertilizer, water stress, soil pH and soil calcium concentration.
Phosphorus The effects of soil phosphorous level on toxic alkaloid production in tall fescue are relatively unknown. A recent study examined alkaloid production in tall fescue grown at low, medium and high soil P concentrations (Malinowski et al., 1998). Alkaloid content usually increased with higher soil phosphorus concentrations. This suggests that high soil phosphorus levels may increase the toxicity of endophyte-infected tall fescue. This is particularly important poultry-producing areas of the country where soil phosphorus concentrations can be extremely high.
More information on the fescue endophyte is available at: Tall Fescue on the web.
Figure 1. Effect of nitrogen fertilization on nitrate accumulation in forages.
(Adapted from Hojjati et al.)
Figure 2. Effect of nitrogen fertilization and plant part onergot alkaloid content of tall fescue.
(Adapted from Rottinghaus et al.)
Hojjati, S.M., T.H. Taylor, and W.C. Templeton, Jr. 1972. Nitrate accumulation in rye, tall fescue, and bermudagrass as affected by nitrogen fertilization. Agron. J. 64:624-627.
Malinowski, D.P., D.P. Belesky, N.S. Hill, V.C. Baligar, and J.M. Fedders. 1998. Influence of phosphorus on the growth and ergot alkaloid content of Neotyphodium coenophialum-infected tall fescue (Festuca arundinacea Schreb.) Plant and Soil 198:53-61.
Rottinghaus, G.E., G.B. Garner, C.N. Cornell, and J.L. Ellis. 1991. HPLC method for quantitating ergovaline in endophyte-infested tall fescue: Seasonal variation of ergovaline levels in stems with leaf sheaths, leaf blades, and seed heads. J. Agric. Food Chem. 39:112-115.
Prepared by John Andrae and Bruce Pinkerton, Extension Forage Specialists at the University of Georgia and Clemson University, respectively.
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