Nutritional interventions for reduced nitrogen excretion
By Marcos Elias Duarte and Sung Woo Kim
In recent years, the increased awareness of public welfare has augmented the demand for sustainable animal production. In the animal industry, sustainability should be driven by efficiency and productivity to archive the best profitability. In the swine industry, nitrogen is one of the main concerns related to public welfare, the environment, and animal health. Nitrogen is excreted mainly in form of urea and microbial protein, among other compounds. The urea generated during animal production can be hydrolyzed resulting in ammonia which represents an agent causing stress and disease in the pigs and to the employees working in the barn.
Additionally, ammoniac nitrogen cannot be recovered as fertilizer. High dietary nitrogen can promote the growth of pathogenic bacteria affecting the intestinal health of pigs. It is well established that a healthy intestine plays an important role in increasing animal productivity and efficiency which in turn may improve the efficiency of nitrogen utilization.
Therefore, nutritional interventions have been accomplished to improve dietary nitrogen utilization to reduce nitrogen excretion and enhance the intestinal health of pigs. Consequently, a healthier intestine would in turn enhance animal efficiency and productivity as well as enhance the efficiency of nitrogen utilization.
Feed formulationAmong nutritional strategies feed formulation is the most practical way to accomplish better nitrogen utilization. Low crude protein diets have been used to reduce feed costs and improve intestinal health by decreasing protein fermentation. It has been estimated that reducing 1% protein in the diet reduces 9% nitrogen excretion and, consequently, environmental pollution.
Protein supplements, generally, present a buffer capacity, consequently, the excess of crude protein in the diet can promote the growth of proteolytic bacteria, including potential pathogens such as Escherichia coli, and Salmonella. Proteolytic bacteria integrate the commensal microbiota of pigs, and they are known to promote immune responses and oxidative stress in the intestine that would later cause damage to the intestinal barrier function and morphology.
The advent of crystalline amino acids brought the possibility to formulate diets with low content of crude protein with adequate and precise amino acid provision. Approximately 98% of the nitrogen in crystalline AA is absorbed by the pigs whereas, compared with crystalline amino acids, protein supplements have lower digestible amino acids and may contain antinutritional compounds, including glycinin and β-conglycinin, that would affect intestinal health, especially in nursery pigs. Nowadays, seven essential amino acids are commercially available which can be used to reduce the crude protein content up to the eighth limiting amino acid if economically viable.
It is worth mentioning that extremely low crude protein diets can affect the growth of even beneficial bacteria due to the limitation of nitrogen. In addition, excessive reduction of crude protein may cause limitation of non-essential amino acids and therefore the supplementation of unspecific nitrogen may be required. Furthermore, reducing the use of protein supplements may reduce the supply of functional compounds such as flavonoids, and functional peptides that are associated with health markers in the intestine.
Therefore, formulating low crude protein diets should account for supplementing all limiting amino acids as well as the consequences of reduced bioactive compounds. Consequently, a precise diet formulation would reduce the excess of nitrogen excretion and promote intestinal health in pigs.
Phase feeding is another strategy to reduce the excretion of nitrogen considering that the provision of nutrients to pigs would be more precise a more phases. However, multiple phases are not always practical in-farm conditions and the economic evaluation should be considered conspiring the health benefits of multiple phased.
Highly digestible ingredientsHighly digestible protein supplements have been used as a strategy to enhance intestinal health and nitrogen utilization. These protein supplements can be divided into three categories: animal-based (mainly blood plasma, fish meal and poultry meal); plant-based (soy protein concentrated, fermented soybean meal and enzymatically treated soybean meal); and microbial-based (yeast protein and Corynebacterium glutamicum cell mass).
The use of these ingredients can reduce the amount of crude protein in the diet and consequently reduce nitrogen excretion promoting intestinal health. Additionally, some of these protein supplements can be considered functional proteins due to the benefits to intestinal health caused by their content of bioactive compounds, including immunoglobulins in blood plasma, and cell wall compounds in single-cell proteins.
Feed additivesFeeding functional feed additives is another strategy to enhance intestinal health and nutrient utilization. Feed additives can enhance nitrogen utilization by directly improving digestibility and by promoting intestinal health which consequently would improve nitrogen utilization.
Selected feed enzymes can reduce nitrogen excretion and improve intestinal health by directly increasing the digestibility of protein or by reducing anti-nutritional factors such as allergenic proteins, phytate and non-starch polysaccharide that can impair protein digestion.
Protease can improve the digestibility of protein reducing the content of allergenic protein from soybean meal and the availability of undigested protein for microbial fermentation. Studies have shown that protease can reduce the concentration of pro-inflammatory cytokines, consequently reducing the oxidative stress status in the jejunal mucosa of pigs. Oxidative stress markers are known to cause damage to the epithelial barrier, therefore, reducing those markers can promote the structure of intestinal epithelium.
NSPase is another important group of enzymes that can improve nutrient digestibility and promote intestinal health. It is well-known that NSP can increase inflammatory cytokines and oxidative stress in the small intestine of pigs. The use of NSPase has been shown to reduce those markers and consequently improve the intestinal epithelium in pigs. Interestingly, the use of xylanase has been demonstrated to reduce oxidative stress in pigs. The reduction in the oxidative stress markers by xylanase can be associated with the reduction of the pro-inflammatory cytokines by reducing the effects of the antinutritional factors; the modulation of the intestinal microbiota toward a healthier microbiota composition; and the release of anti-oxidate compounds.
The growth of fiber-degrading microbiota can increase the microbial protein excreted in feces and in turn reduce the nitrogen excreted in the urine. Microbial proteins are less susceptible to degradation compared with urea from urine. Therefore, switching the nitrogen excretion from urine to feces can reduce the production of ammonia from the hydrolysis of urea.
Phytase can also be associated with the enhancement of crude protein digestibility and intestinal health. In addition to the increased calcium and phosphorus, the use of phytase can increase the digestibility of protein. It is well known that phytate can bind to amino acids and other nutrients reducing their digestibility. The use of phytase can also reduce the use of minerals that contain great buffer capacity. As mentioned above, ingredients with great buffer capacity can promote the growth of pathogenic bacteria. Studies have demonstrated that the use of phytase can enhance intestinal health in pigs by reducing pro-inflammatory cytokines and oxidative stress status. Enhanced intestinal health can also be associated with the modulation of the intestinal microbiota.
Selected organic acids have antimicrobial properties and are used in animal feeds to modulate intestinal microbiota, enhance intestinal epithelial barrier function and improve protein digestibility. Organic acids can reduce the pH in feces and urine reducing the degradation of urea by a microorganism, consequently reducing the production of ammonia. Benzoic acid can reduce the concentration of blood urea nitrogen. The blood urea nitrogen can be used as a predictor of nitrogen content in urine due to their high correlation. Other selected feed additives, including prebiotics, probiotics, postbiotics and phytobiotics can reduce nitrogen excretion by changing the intestinal environment, enhancing the intestinal morphology, immune and oxidative stress status, and microbiota composition. Enhanced intestinal health with increased beneficial microbiota would increase the efficiency of nitrogen utilization consequently reducing nitrogen excretion and consequently promoting the growth of pigs.
In summary, different nutritional strategies can be adopted to improve growth efficiency related to enhancing intestinal health and nutrient utilization, consequently reducing nitrogen excretion. The reduction of nitrogen excretion should consider quantitatively and qualitatively. A qualitative is the shift from urine mainly in the form of urea to feces in the form of microbial protein. Finally, improved feed efficiency and reduced nutrient waste would assure sustainable swine production, economic incentives to producers, and public welfare.
Duarte is a postdoctoral research scholar and Kim is a professor of nutrition and digestive physiology, both in the Department of Animal Science at North Carolina State University.
