Nutritional management to reduce nitrogen excretion

The main source of nitrogen (N) in the diet is crude protein (CP), specifically digestible amino acids, which are nutrients involved in protein synthesis as their main function. Feeds are formulated based on the ideal protein concept, using lysine as the reference amino acid since it is the most limiting one in pigs, and the requirements for the rest of the amino acids are presented relative to lysine.

Protein synthesis capacity is governed by the law of the minimum, with the most limiting amino acid setting the maximum achievable level of protein synthesis. Reducing the CP in a diet while maintaining the same amino acid profile will reduce the excretion of nitrogen into the environment, avoid digestive problems due to excess protein, and at the same time may imply economic savings.

Reducing dietary crude protein content and using essential amino acids are two of the best available techniques (BAT) recognized by Commission Implementing Decision (EU) 2017/302 of 15 February 2017 for reducing excreted nitrogen.

This concept was tested in a trial conducted at the beginning of growing phase (23-45 kg BW) with 1,248 pigs, in which the lysine content in the diet was maintained (1.05% Lys SID, standardized ileal digestible lysine) and the crude protein content was lowered 3%, from 16.5% to 13.1%, thus creating different Lys SID: CP ratios (Table 1).

At the level of digestive safety, with the reduction of CP content in the diet, a linear increase in the probability of hard feces (category 0) was observed, which was assessed weekly from 0 (no diarrhea) to 3 (severe diarrhea), indicating fewer digestive problems due to excess protein. Regarding production results, the increase in the Lys SID: CP ratio (decrease in CP content) increased feed intake linearly while weight gain worsened linearly, especially in treatments 3 and 4.

Using a linear broken-line model, it was determined that the maximum Lys SID:CP ratio to avoid worsening conversion was 6.9%, thus establishing the need for a minimum crude protein content in the diet (Aymerich et al, 2022). With these results, in the nitrogen balance, we observe an 11-12% reduction of N excreted for every 1% decrease in CP in the diet. We also observe an increase in nitrogen use efficiency (retention/ingestion). This is corroborated by the blood analysis results, where a linear reduction in plasma urea content was observed with decreasing CP, indicating less catabolism of excess amino acids.

Table 1. Production results, blood urea content, fecal consistency, and nitrogen balance for pigs at the start of growing phase with different Lys SID: CP ratios.

CP: Crude protein, Lys SID: Standardized ileal digestible lysine, ADG: Average daily gain, ADFI: Average daily feed intake, FCR: Feed conversion ratio.
¹ Category 0 (no diarrhea) - Category 3 (severe diarrhea)
² Considering 16% N in crude protein and 16.75% CP in growing pigs

Given these results, another in-house trial was conducted with two feeding programs throughout the growing- finishing (G-F) period:

• High protein (Entry: 16.6%, growing: 15%, and finishing: 14% CP)
• Low protein (Entry: 14.5%, growing: 13%, and finishing: 12%)

respecting the maximum ratio of 6.9% SID Lys: CP in the low CP treatment with the same lysine levels for both treatments.

The low protein diet reduced pigs' apparent water intake by about 7% compared to the high protein diet consistently throughout the G-F period (Figure 1). The reduced CP content in the diet is associated with a lower potassium content and a lower electrolyte balance since soybean meal, the main source of protein, is rich in potassium. This modification of the electrolyte balance reduces the animal's thirst sensation and decreases water consumption. Consequently, with the low protein diet, the volume of slurry produced during the G-F period was reduced by 11%. The slurry produced was more concentrated with a higher dry matter content and a lower N content in the dry matter (-22%). The overall result is an 18% reduction in the N content of the slurry at the time of pit emptying (Table 2).

Table 2. Slurry production and composition according to high and low protein treatment in the overall G-F period (20-105 kg LW.).

In conclusion, reducing the CP content in the diet of finishing pigs reduces apparent water consumption, generating less slurry volume per pig. We also guarantee optimal digestive safety, reducing protein excess and the catabolism of excess amino acids, therefore reducing the N excreted and the N to be managed in the slurry.