Accumulation of excessive nutrients from poultry litter applied to Mississippi farms may result in a nutrient imbalance that can cause pollution. Using efficient feedstuffs and feeding techniques can significantly reduce the nutrient content of excreted manure and help reduce odours and other gaseous emissions from manure (Sutton and Lander, 2003).

Properly managed poultry diets help balance nutrient flows and reduce the potential negative impacts some nutrients may have on the environment. Managing poultry litter and diets will become increasingly important in the future because many US states have chosen to address the nutrient flow issue by placing restrictions on land-application of poultry litter to soils considered high in phosphorus.

Phosphorus and Poultry Litter: The Issue

Plant growth depends on an adequate supply of both phosphorus (P) and nitrogen (N) in the soil. However, many crops require only one-third to one-fourth as much phosphorus as they do nitrogen for optimum yields (Hansen et al., 2005). Because poultry litter contains similar amounts of nitrogen and phosphorus, the potential to over-apply phosphorus existed in the past if litter application rates were based on crop nitrogen needs.

Unfortunately, limiting litter applications to crop phosphorus rates means there is a lot of extra poultry litter generated in Mississippi today. In addition, current phosphorus restrictions are forcing many producers who had previously used poultry litter for its nitrogen to have to purchase additional commercial nitrogen fertiliser to meet crop nitrogen needs, thereby increasing production costs. Soil type (sand vs. silt vs. clay) may have an impact on nutrient retention properties of soils and, thereby, application rates of poultry litter and commercial fertiliser.

Poultry diets are made up primarily of plant seeds - usually corn and soybeans. Corn and soybean meal comprise as much as 80 per cent of poultry diets. However, much of the phosphorus is bound in the phytate-phosphorus form and unavailable to the bird.

Phytate-phosphorus within a given feedstuff is variable but typically averages 72 per cent of the total seed phosphorus in corn and 60 per cent in soybean meal (Ravindran et al., 1995). Because poultry digestive systems lack the phytase enzyme that breaks down and releases phosphorus, only 10 to 30 per cent of the phosphorus in corn and soybean meal is available to poultry (Sims and Vadas, 1997). Therefore, forms of inorganic phosphorus, such as defluorinated phosphate, monocalcium phosphate and dicalcium phosphate, are often added to the feed to meet dietary requirements.

The combination of unavailable phosphorus from grain sources and added inorganic phosphorus means that much of the total phosphorus passes through the birds and ends up in the litter, increasing litter phosphorus concentrations (Hansen et al., 2005). As a result, land application of litter over a number of years may contribute to the build-up of phosphorus in some Mississippi soils.

Mississippi’s poultry industry is vital to the state’s economy and to many local agricultural communities across the state. Virtually all of the litter produced by the poultry industry in the state is land-applied as a nutrient source for forage or row crops. However, there is increasing concern that soils with excess phosphorus accumulation may lose phosphorus via run-off, erosion and subsurface drainage. If this run-off enters surface waters, it can contribute to eutrophication and associated water quality problems such as algal blooms, fish kills, odours, scums, turbidity and sedimentation (Sharpley et al., 1994).

As a result, there is now considerable interest in developing agricultural management practices to prevent phosphorus accumulation in soils and phosphorus transport from soil to surface waters in the state.

Enzyme Use in Poultry Nutrition

The primary way to reduce the amount of nutrients excreted is to decrease the amount consumed and improve nutrient absorption (Sutton and Lander, 2003). The quantity of nutrients excreted by animals is affected by three main factors:

1. amount of dietary nutrients consumed
2. efficiency with which these nutrients are used by the animal for growth and other functions, and
3. amount of normal metabolic losses (endogenous)

In other words, the amount of excreted nutrients can be expressed as:

Nutrients excreted = nutrient intake - nutrients used + endogenous nutrients

There is increasing interest today in using enzymes, genetically-modified feed ingredients and feed processing technologies to enhance the availability of nutrients so as to meet the specific needs of the bird and reduce nutrient excretion.

Enzymes are proteins or protein-based substances that speed up or catalyse chemical reactions (Applegate and Angel, 2004). Phytase enzymes convert phytate-phosphorus to inorganic phosphorus so that it is more readily available to poultry. Therefore, when diets are formulated with phytases, a certain amount of inorganic phosphorus can be removed from the diet (Applegate et al., 2008). Otherwise, the soluble portion of the phosphorus in the diet will actually increase as a result of the additional inorganic phosphorus not needed by the animal.

The amount of inorganic phosphorus to be removed from the diet formulation will depend on how much phosphorus is being fed over and above the animal’s requirement (Applegate et al., 2008).

Phytase is present in microorganisms, plants, and certain animal tissues. Phytase present in the digestive tracts of poultry can potentially come from four sources:

1. phytase native in digestive secretions
2. phytase native in feed grains
3. phytase from bacteria native in poultry or feed grains
4. phytase produced by microorganisms and added to the feed

The first three phytase sources are negligible in poultry. Therefore, to increase phytate-phosphorus availability in poultry feed, the phytase enzyme must be produced by microorganisms and added to the feed as a supplement (Sims and Vadas, 1997). For example, inclusion of fungal phytase in poultry diets improved phosphorus retention by broilers from 50 to 60 per cent (Kornegay, 1999; Simons et al., 1990).

However, efficacy of phytase supplementation depends on numerous factors, including microbial source, form of the enzyme (coated, particle size, and so forth), temperature, pH of the enzyme, diet mineral concentration, ingredients used in the diet, diet manufacturing methodology, form of the diet (pelleted, mash, or liquid), location of addition of phytase (post-pelleting or mixer), type and level of vitamin D metabolites, disease status of the animal, and other factors (Ravindran et al., 1995).

Enzymes are unique in that they are highly selective for the molecules they act upon (substrates) and for the end-products they produce. For an enzyme to work, it must be in close proximity to the substrate, and the action site must not be blocked (Applegate and Angel, 2004).

Enzymes are proteins and therefore can be denatured or destroyed by anything that can change their structure. For example, the inability of dietary phytase to survive pelleting and storage temperatures contributes to the low efficacy of phytase application (Slominski, 2011). To counteract these negative temperature effects, post-pelleting applications, granulation and enzyme coating have been proposed. Even then, phytase must maintain functionality in the digestive system of the bird (Loop et al., 2012).

Phytase function may be affected by pH of the gastrointestinal tract and gut transit time. This means that the pH activity range of the phytase, relative to broiler physiology, should be an important factor in choosing a commercial phytase, as it will determine to a great extent how well the enzyme works under different physiological conditions and feed nutrient concentrations (Angel, 2006).

It is believed that phytate hydrolysis (decomposition of a chemical compound by reaction with water) takes place mainly in the crop, proventriculus and gizzard, where the pH is more conducive to phytase action and the phytate is more water-soluble (Selle and Ravindran, 2007). However, in other areas of the gastrointestinal tract, the phytate molecule may actually bind zinc, copper, manganese, iron, magnesium, calcium and cobalt, among others, in very stable complexes (Angel, 2006). The availability of any nutrient held in these complexes is greatly reduced. In addition, these phytate complexes tend to precipitate at the higher pHs found in the small intestine, and most enzymes have little to no effect on these precipitated complexes (Angel, 2006).

This will likely be an important consideration in the future as more states pay additional attention to the build-up of copper, zinc and other elements in soils amended with poultry litter (Sims and Vadas, 1997).

Fortunately, a reduction in litter phosphorus can be achieved without any adverse effects on poultry production, bird health or carcass quality (Hansen et al., 2005). However, in assessing the efficacy of phytase, Slominski (2011) indicated consideration should be given to the large granule size of some phytase products, which may prevent the substrate phytate from being hydrolysed effectively in a relatively short period of time in the critical compartments of the upper gut. This could be the result of uneven enzyme/substrate distribution within the feed matrix and subsequently in the digesta, which, in turn, could delay phytate-phosphorus release (Slominski, 2011).

Nutrient Balance

As mentioned earlier, without careful attention, one possible source of some 'high-phosphorus' soils could be surplus phosphorus applied to fields on poultry farms or other locations, where phosphorus inputs (feed, litter) have historically exceeded phosphorus outputs (birds, harvested crops and/ or forage) (Hansen et al., 2005).

Without proper management, the excess phosphorus in poultry litter that is land-applied over several years may eventually cause a build-up of soil phosphorus.

Nutrient 'imbalances' are known to occur where animal production is geographically concentrated (Beegle et al., 2002; Slaton et al., 2004). This makes achieving a nutrient balance a critically important goal for the long-term, sustainable management of poultry litter in Mississippi. It is important for all producers who use poultry litter on their farms to follow best management practices for litter application regardless of whether or not they raise poultry or where their farm is located within the state.

Better nutrient retention by the bird and continued advances in nutritional programmes are important steps toward reaching nutrient balance on individual farms and in Mississippi as a whole. Sutton and Lander (2003) reported dietary considerations that offer the opportunity for reducing nutrient content of poultry litter have several advantages:

1. a smaller land base per animal unit is required for litter application
2. a greater volume of litter can be applied per acre of land to meet agronomic crop production rates, and
3. reduced nitrogen and sulphur excretion may reduce the amount of odour associated with litter application.

As nutrient balances improve, challenges will still remain regarding 'high-phosphorus' soils and how best to manage them. However, use of phosphorus index equations and nutrient management plans can help identify fields with the greatest potential for phosphorus run-off.

In addition, increased use of best management practices (i.e. improved litter application and storage practices, buffer strips, and so forth) will help lessen the threat of phosphorus run-off to surface waters.

Summary

To ensure the long-term sustainability of poultry production and environmental quality in Mississippi, disposal of poultry litter must be accomplished in an environmentally sound manner.

Phytase is a cost-effective enzyme that improves phosphorus use from phytate-phosphorus in feed grains and can help decrease phosphorus levels in litter. However, the bird consumes considerably more phosphorus than it uses, so there is plenty of opportunity for improvement in phytate-phosphorus release and increased phosphorus retention in poultry flocks.

Options such as phytase use and low-phytate feed grain sources can help lessen the amount of phosphorus excreted in the litter and reduce the potential for poultry litter to contribute to surface run-off and non-point source pollution of Mississippi’s surface water resources.

November 2013