Nutrients supplied in excess via feed are excreted by the pig.¹ Therefore, a diet with reduced nitrogen (N) and phosphorus (P) content (N/P-reduced feeding) is a suitable method for reducing emissions in pig farming.³ Most nitrogen emissions result from an excessively high proportion of crude protein (CP) in the diet.⁴

As pigs do not require crude protein but rather essential amino acids, the supply of essential amino acids is central to modern feeding concepts. It is important to note that amino acids are the smallest building blocks of proteins. Essential amino acids are vital for life but cannot be produced by the animal itself and must therefore be supplied via the diet.⁵ Accordingly, it is possible to reduce crude protein in the total diet down to the point where the first essential amino acid would become deficient at lower levels. By adding these essential amino acids via the feed, an adequate supply of amino acids is ensured even with a low crude protein content.⁶ Nutrient-reduced feeding methods (DLG: very strongly N/P-reduced) with the increased use of free amino acids can reduce N excretion and ammonia emissions by more than 20%.⁷

The greatest emission-reducing effect of feeding is achieved when free amino acids and high-quality or processed proteins are used. Feed costs may vary depending on the additives used. Undernutrition of the animals must be avoided, as this can lead to health problems. An oversupply resulting from safety margins should be avoided, as this could otherwise lead to an excess of protein, which would counteract emission reduction. To feed the animals according to their needs without creating surpluses, the farmer or animal caretaker should be aware of the animals’ performance and the exact composition of their feed.
¹
Feeding practices for breeding sows, fattening pigs and piglets are categorised into four groups based on N/P reduction. These are:

1) Universal fattening or standard feeding without reduction measures,

2) N/P-reduced,

3) highly N/P-reduced,

4) very highly N/P-reduced feeding.
 

Table 2 presents DLG recommendations for nutrient-reduced feeding in pig fattening for groups 3 and 4.

The lower the N and P contents, the more important it is to prevent deficiencies, nutritional imbalances and reduced performance in the animals through adapted feeding management, as the reduction of N and P in the rations also results in a reduced protein supply. Feeding with very low nutrient levels and the use of free amino acids can reduce N excretion by up to 20%. Regular performance checks and adherence to current nutritional recommendations must be taken into account. Feed analyses should be carried out for both home-grown and purchased feed.⁷
A reduction of just 1% in protein in the ration can reduce NH₃ emissions in pigs by 10–11%.⁸ Whether the reduction in nutrients also offers economic benefits must be assessed on a farm-by-farm basis, as the costs of free amino acids and protein sources vary considerably.
¹
Numerous trials have shown no deterioration in biological performance or slaughter performance with feeding regimes featuring a significant or very significant reduction in N and P.^9,10,11 In a trial conducted by the Schwarzenau Research and Training Centre with breeding sows, no negative effects on biological performance or animal health were observed despite a significant reduction in nutrient levels below the level of very severely N/P-reduced feeding during gestation and lactation.¹² A reduction in crude protein content of up to 4% is possible in the fattening phase without compromising carcass quality.¹³ According to the DLG, reducing nutrient levels below those of very severely N/P-reduced feeding regimes can, in some cases, lead to a significant increase in feed conversion (kg of feed per kg of weight gain) during the fattening phase.¹⁴
The Lower Saxony Chamber of Agriculture found in a trial that feed consumption decreases in animals receiving a N/P-reduced ration. One consequence of this is a reduced volume of slurry.¹⁵

In trials conducted by LUFA Nord-West to measure emissions in the ‘transparent barn’ in Quakenbrück, three feeding trials were carried out in which crude protein levels were below the DLG guidelines for very low N/P diets (16.5% CP up to 60 kg DM, 14.0% CP up to 80 kg DM, 12.0% CP from 80 kg DM). The control group received a universal feed with 16.5% CP. Ammonia emissions were reduced by an average of 0.65 kg per fattening place per year (22% compared to the control group). In these trials, feed consumption in the experimental group was significantly higher than in the control group.¹⁰
 

The reduction of N and P in the ration can be achieved through phased feeding. In this approach, animals in piglet production and fattening receive rations adapted to their live weight and performance level.1 This measure serves to reduce crude protein in the feed ration. Emissions can be reduced with more feeding phases; however, phase feeding must also be accompanied by a corresponding reduction in crude protein in the individual feeding phases. In fattening pig farming, a minimum of three-phase feeding in accordance with the TA Luft is mandatory for herds of 1,500 fattening pigs or more. The higher the feed intake, the more protein feed can be saved, which is why phase feeding is particularly recommended during the final fattening stage.¹

When reducing nutrient content, it must be noted that increased feed intake does not achieve any effect if the nutrient content is reduced. Detailed information can be found in DLG Fact Sheet 418. In addition, there is an Excel tool that provides a calculation aid for determining individual farm feeding regimes with regard to the maximum crude protein and phosphorus levels to be adhered to in N/P-reduced feeding.

Excel tool for N/P-reduced feeding

N/P-reduced feeding is not feasible for organic farms, as free amino acids and phytases are currently not permitted for use there.¹⁶

The reduction in emissions achieved through nutrient-reduced feeding is limited, but can be supplemented by other technical methods for emission reduction. In addition to the ingredients of the feed, attention should also be paid to feeding techniques. For example, feed residues falling into the slurry promote emissions.

The nutrient-reduced feeding method for breeding sows and fattening pigs complies with BAT (Best Available Techniques) and the state of the art as defined by the TA Luft (Technical Instructions on Air Quality Control) and can achieve a reduction of up to 40%.¹⁷ For multi-phase feeding, subsidies are available in some cases through the Agricultural Investment Support Programme. Further information on this and other eligible measures can be found in the KTBL brochure ‘Eligible techniques for emission reduction in livestock buildings’.