Nutrient Cycling in Pastures

by Joao Vendramini

published in The Florida Cattleman and Livestock Journal

Warm-climate grasslands are generally characterized by extensive grazing systems with limited (or no) use of commercial fertilizer and supplemental feed. Extensive areas of planted pastures are degraded due to inadequate supply of nutrients, particularly nitrogen. Nitrogen is the most limiting nutrient for forage production. Low levels of nitrogen in the soil associated with high cost of commercial fertilizer, and limited use of nitrogen-fixing warm-season legumes are some of the factors contributing to inadequate supply of nitrogen to grass pastures. Optimizing nutrient cycling represents a viable alternative to enhance nitrogen availability to forage crops and, consequently, maintain the productivity and long-term sustainability of low-input pasture systems. Nutrient removal from grazing systems as animal products is generally small compared to harvested forage systems such as hay fields. Nutrient retained in cattle body tissue and exported through beef or milk generally represent less than 30% of total nutrient ingested by the animal. In grazing systems, the majority (70% or more) of the ingested nutrients is returned to the pasture via excreta (dung and urine). Conversely, in harvested forage systems where nutrients are not returned to the soil because forage is fed to livestock elsewhere, nutrient removal rates can be significant. The proportion of the nutrients not harvested or consumed returns to the soil via deposition of senescent plant material. Thus, excreta and litter consisted of the two major pathways for nutrient cycling in grasslands.

Nutrient cycling via plant litter

Plant litter is an important source of nutrients in forage systems. Because of lodging, and trampling of aerial plant parts, litter decomposition is a fundamental process influencing nutrient cycling in pastures. Accumulation of plant litter may also affect plant composition and persistence. 

In well-managed grazing situations, the proportion of the leaf tissues escaping the intake by animals is approximately 30% of the leaf tissue produced. A study conducted in Gainesville evaluated the effects of increased stocking rates and nitrogen fertilization levels on litter mass, deposition rate, and chemical composition in continuously stocked bahiagrass pastures. Greater stocking rates (3.2 AU per acre) and nitrogen fertilization (nitrogen levels up to 320 lb/acre) initially resulted in less litter mass because of greater consumption of the produced forage by the animals. However, later in the growing season, greater litter mass was observed in response to more intensive stocking rate and nitrogen fertilization level. More intensive grazing systems (high stocking rates and nitrogen fertilization levels) also resulted in smaller litter deposition rate and greater litter nitrogen concentration as compared to low-intensity systems. Litter of tropical grass pastures is hard-to-decompose due to its chemical characteristics. One of the factors that affect litter decomposition is the carbon to nitrogen (C:N) ratio of the plant material. Because warm-season grasses normally exhibit low tissue nitrogen concentration, the C:N ratio is greater than cool-season species. Under high C:N ratio (> 30), litter decomposition “competes” with pasture for nutrients. This process is known as nutrient immobilization and it is often associated with pasture decline. Low nitrogen availability can result in nutrient deficiency and subsequent decrease in forage production, nutritive value, and pasture persistence.

The use of warm-season legumes as a means to improve litter quality

Warm-season grasses produce low quality litter, which may lead to immobilization and nutrient deficiency in low-input pasture systems. An alternative to minimize the negative effects of nutrient deficiency is to improve litter quality and, subsequently, promote more efficient nutrient cycling in pastures. Mixed grass-legume pastures represent a feasible alternative to improve litter quality. Nitrogen-fixing legumes can fix nitrogen from the atmosphere that can later be utilized by the grass. Three conditions are necessary for efficient nitrogen fixation in mixed swards: i. high forage production and ii. high proportion of legumes in the mixture (> 50%). Maintenance of sufficient legume populations has been difficult in many pastures due to selective grazing, inadequate soil fertility, and stand decline caused by pest infestation. Nitrogen fixed by the legume is transferred to the soil and subsequently to the forage grass via decomposition of above and below-ground legume residues and animal excreta.

Nutrient cycling via excreta

Approximately 70 to 90% of the nutrients enter the pasture via excreta deposition; however, nutrient return through excreta is not uniform. In general, a cow deposits 11-16 dung and 8-12 urine events per day with approximately 6 lb of wet dung and 60 fl ounces of urine per event. One urine event supplies the equivalent of 400 and 800 lb/ac of nitrogen and potassium, respectively, in the area covered by the event (approximately 1 ft2). 

Cattle tend to congregate in small areas of the pasture, usually near shade, watering, and resting sites. In addition, topography and other site conditions can exert differential effects on grazing and excretion behavior, leading to zonal heterogeneity in nutrient distribution. Cows usually prefer flat zones as resting sites and visit inclined zones only for grazing. Thus, a greater proportion of excreta return occurs on these areas, reducing soil fertility in other pasture areas and increasing nutrient concentration at the resting sites. A study was conducted to evaluate nitrogen and phosphorus deposition patterns by cattle in an established pasture with no history of fertilizer application and more than 50 years of grazing. Nutrients accumulated in the flat topography zone and were depleted in the steeper areas, resulting in heterogeneous forage production, nutritive value, and stand persistence across the landscape. Partitioning of nutrients between feces and urine also reduces the uniformity of nutrient return through excreta. While phosphorus, magnesium, and calcium are returned to the soil mainly with feces, urine contains significant amounts of potassium and sodium. Because dung and urine events often do not occur at the same site, nutrient return is not uniform. 

Grazing management exerts a major influence on herbage nutrient returns, altering the proportion of nutrients returned via excreta and litter. Stocking method is one management option to improve nutrient distribution. Rotational stocking with short grazing periods enhances uniformity of excretal return while reducing steep nutrient gradients near shade and watering sites. In Florida, dung distribution of heifers was evaluated under two rotational stocking strategies (7-d and 1-d of grazing period) or continuous stocking on Pensacola bahiagrass pastures. Rotational stocking with a 1-d grazing period promoted a more uniform dung distribution compared to rotational stocking with 7-d grazing periods and continuous stocking. 

Animal aspects that influence solar radiation tolerance may be considered in warm climates as tools to enhance nutrient distribution. Cattle breed and coat-color may interact with environmental conditions and, thereby, affect pasture utilization and nutrient redistribution patterns. In this aspect, Brahman cattle spent less time under shade than non-Brahman cattle and Holstein cows with predominantly black coats spent 20 min/day more time under shade in Florida compared to predominantly white-coated cows. Because there is a correlation between time spent in a particular pasture area and the number of excretions, the more time the cattle spend under the shade, the greater is the nutrient concentration in that area (i.e., less uniformity of distribution). Increasing air temperature and temperature-humidity index increased time spent under the shade and affected patterns of excreta return. 

It is frequently observed that grazing animals avoid fouled herbage if they have the choice and this behavior may lead to creating heterogeneous herbage mass and nutritive value in the pasture. In Florida, urine and feces from heifers grazing fertilized bahiagrass pastures were collected and reapplied in the pasture to measure the impact of dung patches and urine spots on forage production. In urine-treated plots, herbage accumulation increased from ~ 2500 at zero application to ~ 4400 lb DM/ac with three applications of urine. Herbage response was generally greatest near the center of the urine deposit and decreased as distance from the center of the urine application increased. In dung-treated plots, physical interference by dung resulted in decreased herbage accumulation in the area under the dung deposit but no effect was observed in areas not affected by dung deposits.

Conclusions

Unfavorable economic return of fertilization and depleted soil fertility conditions in extensively-managed beef cattle operations reduce warm-season grass pasture productivity and persistency. As a result, producers often reduce stocking rates and animal productivity in response to poor pasture performance, which may impact the profitability and economic returns of livestock operations. Management practices such as stocking method, grazing intensity, strategic fertilization, and overseeding warm-season legumes are alternatives that can promote nutrient cycling in warm-season grass pastures and enhance soil fertility conditions and, consequently, improve forage production and persistence.