by David Serene
Green House Gaz (GHG), heavy metals, drugs residues, pathogenic bacteria or smells are critical issues that farmers need to get acceptance for their manure as ‘green’ fertilizer. To upgrade manure quality, we can act both by adjusting feed diets distributed to animals and by upgrading sludge processing.
Upgrading fertilizing value of manure
Nitrate and ammonium are by-products of the degradation of the protein. To reduce the production of these gas, we need to reduce the excess of protein in the animal diets and increase the digestion and absorption of these proteins by animals. That will require from nutritionist to reduce the minimum crude protein level and focus more on digestible level of amino-acids. Based on this principle, Korea has recently changed regulations. Instead of setting a minimum level of crude protein, they decide to enforce a maximum level of crude protein to limit the excess of protein in animal diets that would end up as excess of nitrates and ammonium in soils.
To enable the digestion of proteins, we need to activate the pepsinogen into pepsin through carboxylation. This carboxylation process occurs only at low pH, below 3.5. But when the diet enter the stomach, it will trigger an increase of the pH. The stomach will produce more HCl to get the pH back below 3.5 as soon as possible. But the use of acid buffer as Carbonate Calcium, Sodium Bicarbonate or Zinc Oxide have a detrimental effect on the stomach pH. At high level, these components will bind HCl, and lead to an increase of stomach pH which affect negatively the digestibility of protein. To avoid the restriction of pepsinogen activation, we are recommending to reduce the dosage of Carbonate Calcium, Sodium Bicarbonate and Zinc Oxide and to add formic acid to the diets in order to drive back the pH below 3.5 as fast as possible. But to have a real effect on stomach pH, formic acid must be used at 5 to 7kg per ton (of pure formic acid). This is higher than the level currently used in commercial animal diets.
Regarding Phosphate soil contamination, we must have a look at the phosphorous bound in the phytate complexes. To digest the phytates and release the phosphorous for maximum absorption, we recommend to use phytase enzymes. But at the same time, we must avoid high dosage of Zinc and Copper. Indeed, when in excess, Copper and Zinc will attach to the phytate complexes and prevent phytase enzymes to work efficiently.
Through a proper strategy of phosphorous optimization, we can reduce phosphorous pollution by 50 to 70%.
Zinc and Copper are commonly used in Swine and Poultry nutrition at high level as growth promoter. Indeed, the positive effect of both Zinc and Copper on gut flora regulation and gut integrity have been scientifically demonstrated. But once in large quantity in the manure, the Zinc and Copper will still demonstrate an antibacterial activity that would heavily disturb the microbiological biotope of the soils.
Morever, recent publications underlined the effect of high level of Zinc and Copper on the development of bacterial resistance. We even demonstrated that these resistances can be crossed with antimicrobial as bacitracine and chlortetracycline. This selection of super-resistant strains of bacterias starts inside the gut of the animals but will occur as well in the soil where zinc and copper will meet bacterias. It is becoming critical to reduce the level of zinc and copper currently used in animal diets to reduce contamination of the manure and facilitate a safer use as fertilizer. For these reasons, several countries already regulated the maximum acceptable level of copper and zinc in animal diets and more countries will follow in the coming years.
Other contaminants as heavy metals, antibiotic residues or presence of pathogenic bacteria should be minimized as well to limit soil pollution.
Upgrading anaerobic treatment of manure for optimal production of energy
Manure is indeed an important source of energy. All the nutrients present in the diets have not all been used by the animal and some more energy can be extracted through anaerobic fermentation. The fermentation will produce methane that can be burnt into heat to feed an energy converter. But as for fertilizer, contaminants present in the manure could negatively affect the fermentation and limit the extraction of energy.
All elements that could hurt the development of anaerobic bacteria should be limited. High dosage of copper and zinc or antibiotics demonstrated a depression of the methanogenic bacteria and a reduction of the production of methane in the biogas.
To optimize the manure value through a biogas and fertilizer strategy, nutritionists need to take the above elements into consideration when formulating diets. Some arbitration regarding the use of some additives must be well thought in order to optimize the utilization and marketing of the animal manure. The value of manure in the farm business model is limited today but it could increase and become a significant source of revenue.
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