Nutrient excretion

Animals are not and will never be 100 percent efficient at utilizing their feed for maintenance and growth with zero excretion, nor should we expect them to be, since humans are far less efficient. Therefore, no matter what we feed animals, there will be manure (a combination of feces and urine) produced. This is often referred to as “waste” in the popular press, but in fact, animal manure is a valuable fertilizer. We grow all sorts of plants, from corn to tomatoes. When we harvest all or part of these plants, we often overlook the fact that in order for that plant to grow, nutrients were removed from the soil, which need to be replaced. These nutrients are often replaced with commercial fertilizers. However, animal manure is also a great way to replenish the lost nutrients, and has the added benefit of having additional organic matter as well. So, why are there concerns? There are legitimate concerns with animal manure and the environment.  Animal manure has to be properly managed. If it is over-applied, meaning that more nutrients from manure are being applied to the land then will be removed by the crops grown on the land, nutrients can build-up in the soil and potentially enter waterways via leaching or erosion events.

Leaching refers to a nutrient's ability to move downward through the soil. This occurs primarily with nutrients that loosely adhere to soil particles. Nitrogen (N) is a good example. Over time, nitrogen will leach through the soil and can enter the water table.

Erosion occurs when soil is physically displaced by water. We often think of erosion along the coast or in fast moving rivers. However, erosion can happen as a result of a heavy rain even from a farm field. This is more likely to happen in barren fields that don’t have plant root systems to help hold the soil in place, similar to what you would find after the field is tilled in preparation for planting. If the soil is eroded, any nutrients in that soil are also eroded and have the potential to enter into waterways. Phosphorus (P) is a good example of this. Phosphorus fairly tightly adheres to soil particles and therefore does not move through the soil (by leaching) the way nitrogen does, but it can be lost in an erosion event.

Why do we want to avoid nutrients from entering waterways of the U.S.?

The best, publicized reason for the detrimental effects of excess nutrients in waterways of the United States is algae blooms, which lead to eutrophication. Algae, just like any other living organism, needs certain nutrients to survive and grow. One of the major nutrients needed for growth of algae is phosphorus. If excess phosphorus is supplied in the water, the algae grow at a rapid rate. This results in two major problems. First, as the algae grow, they block out the sunlight, which can result in the death of sub-aquatic vegetation, which help replenish dissolved oxygen concentrations in the water. This problem might be somewhat offset by the fact that algae also produce oxygen, except for the fact that algae also die off at a rapid rate. As they do, they are decomposed by bacteria, which use a large amount of oxygen in this process. This results in reductions in dissolved oxygen concentrations in the water, which in severe cases can result in the death of larger aquatic organisms like fish. This problem was best illustrated in the 1960s and 1970s when phosphates were used in laundry detergents and more recently where phosphates in dishwater detergents have been linked to massive algae blooms.

Can threats be minimized or avoided?

Realistically, no threat can be completely avoided, but they can certainly be minimized. In the U.S., there are now national standards that regulate the amount of N and P from animal manure, which can be applied to cropland based on soil type, nutrient profile and crop nutrient removal rates. Many individual states have used these national standards as a baseline and have opted for more stringent regulations. 

Structure of Phytic Acid

While regulations are one way to enforce compliance with environmental standards, perhaps more exciting are the changes that animal producers have put in place on their own to alter nutrient excretion profiles and minimize any potential environmental threat. One of the major problems with swine manure and poultry excreta was that the concentrations of nitrogen and phosphorus were not in the correct ratio, relative to the needs of most crops.  Therefore, if swine manure was applied to a field of Bermuda grass hay to meet the nitrogen needs, approximately three to four times the P requirement was applied, which resulted in phosphorus accumulation in the soil over time. One of the primary reasons for this imbalance in N and P was because 60 to 80 percent of the P found in plants is bound in a storage from know as phytate. This phytate molecule cannot be broken down by monogastrics (simple stomached animals: humans, swine, poultry, etc) and therefore, the phytate bound P is not available to the animal and is excreted. In the late 1950s, researchers reported that by adding the enzyme “phytase” to poultry diets, that much of the phytate P could be releases and made available to the animal, thus reducing the excretion of P. Unfortunately, at this time, the enzyme had to be extracted from plant material, which was an extremely expensive process. Over the next several decades, advances in science led to the ability to produce phytase from fermentation of yeast, making it cost-effective to add phytase to swine and poultry diets. This has resulted in about a 30 percent decrease in P excretion.  

Interestingly, work with the enzyme phytase has also had marked effects in better nutrition for humans in developing countries. Diets in impoverished countries tend to be more vegetarian-based. One problem with this is that a large amount of phytate is consumed.  Phytate, not only binds the P contained within its structure, but also has the capability to form salts with positively charged minerals, rendering them unavailable for absorption. High phytate diets have been linked to Zinc deficiencies in humans. Adding phytase to the diet of humans consuming high phytate diets is one method of preventing this deficiency.

Another contributing factor in the imbalance of N and P in animal manure arises from the fact that N in manure has the ability to volatilize to ammonia and be lost from the soil. This is particularly true if the manure is applied on top of the soil.  Topical application use to be the primary mode of application. If the soil was rapidly tilled after application, than a majority of the N would be retained in the soil and hence beavailable to the plant. However, if it were allowed to sit, virtually all of the N would be lost as ammonia within a week. Therefore, much research has been done investigating sub-surface application of manure, and best practices now include injection of animal manure under the soil, preventing ammonia volatilization.

Are large farms a bigger environmental threat?

The number of animals raised on farms has increased over the last 50 years for all species. The reasons for this are fairly simple: the profit per animal has decreased, and there are inherent cost-savings as a result of scale. So, while a pork producer may have been able to support a family of four, 50 years ago with 100 sows, that same farmer would probably need closer to 2500 sows to support that family of four in today’s world. Large farms, sometimes dubbed “Factory Farms” have come under a lot of scrutiny with respect to their potential impact on the environment. This makes sense, because the more animals you have in one location, the more manure is produced. From a shear quantity standpoint this presents a greater potential environmental risk. However, this scrutiny has resulted in heavy regulation of farms raising large numbers of animals. These same regulations are not enforced on smaller farms.  So while the volume of manure and quantity of nutrients produced on larger farms does at first seem to present the greatest environmental risk, it is these large farms that are under the most stringent environmental regulations in an attempt to minimize any potential risk.

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Video describes how swine manure is utilized as a fertilizer and feed into the life cycle of the farm.

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