Practical solutions for environmentally friendly pig production – reducing nitrogen excretion by reducing dietary crude protein - Article 2

7th February 2024

Practical solutions for environmentally friendly pig production – reducing nitrogen excretion by reducing dietary crude protein - Article 2

Local Research Group explores ways to reduce nitrogen excretion

This is the second article in a series by the Pig Research Consortium between AFBI, Thompsons, Devenish Nutrition Ltd and Preferred Capital Management. In our first article, we suggested that pig producers should not phase feed in the late finishing stage as the impact of the diet change reduces production performance and therefore the lower dietary crude protein (CP) in the later stage has no beneficial effect on reducing nitrogen (N) excretion. We also indicated that boars and gilts should be housed separately, and boars should be offered a 15% CP and gilts a 13% CP diet, respectively. This practice will not only improve pig flow on-farm, but it will also reduce N excretion from finishing gilts by 18.5%.

The pig industry has been proactive in responding to environmental challenges by reducing CP in diets from a historical high level and significant reductions in nitrogen excretion have been achieved in recent years with the adoption of our research. More recent work by the Pig Research Consortium has focussed on the finishing diet where lean deposition is beginning to reduce, therefore the late finishing diet is the one with potential for the lowest CP content to reduce nitrogen excretion. Our work has highlighted the requirement to maintain performance and to carefully consider the formulation of diets in terms of balancing for amino acids. If this is done, there is the potential to reduce dietary CP, even within finishing diets for boars. We have established that a 13% CP diet containing 0.9% total lysine was not adequate for boars from 60 kg. However, as amino acids are the essential component and not CP per se, our next step was to test if we could “go lower” by conducting a trial with boars and gilts offered either a 13% CP containing either 0.9% or a higher level of 1% lysine (and correspondingly higher levels of other essential amino acids). We compared the performance of boars and gilts from 60-115 kg offered these diets with those offered the 15% CP/1% lysine control diet.

Table 1. The effect of gender and dietary lysine level of finishing pig performance and N excretion

Table 1 shows that the performance of boars offered the 13%/ low lysine was significantly lower than those offered the 13%/higher lysine diet (1.18 vs. 1.27 kg/d growth rate and 2.37 vs. 2.18 FCR). This drop in performance would reduce return over feed costs and increased nitrogen excretion by 20% which again stresses the need to maintain production performance to minimise emissions.

Table 2. The effect of gender and dietary CP level on finishing pig performance and N excretion

Interestingly, when the performance of boars offered the 13%/higher lysine diet was compared with those offered the 15% CP diet, there was no difference in growth rate or FCR indicating that the 13% CP/higher lysine diet was adequate to drive optimum performance (Table 2). As a consequence of the similar performance and the reduced nitrogen intake of pigs offered the 13% CP/higher lysine diet, nitrogen excretion was reduced by 23% when compared with pigs offered the 15% CP diet. This is a substantial reduction and can be extrapolated using current pig numbers in Northern Ireland to have the potential to reduce nitrogen excretion by 499t/ annum.

However, while the thought of moving all finishing pigs from 60 kg upwards to a 13% CP diet is tempting in our drive to reduce nitrogen excretion, certain caveats must be highlighted, or nitrogen excretion could increase rather than decrease. One reason that the Pig Research Consortium would not recommend blanket reduction to 13% CP in later finishing diets is the variability in intake and growth rate across farms. It is well known that some farms support higher levels of intake and growth than others and it is these farms that could potentially reduce to 13% CP/higher lysine diet. Offering this diet on farms where intake is lower, would limit the intake of lysine and other essential amino acids causing a further reduction in performance and more nitrogen excretion as we observed when we offered the lower lysine 13% CP diet to boars. Our recommendation is to understand your on-farm performance, and then with careful formulation, dietary CP could potentially be lowered to below 15% for boars in the late finishing stage. Finishing gilts can be offered a 13% CP diet with no adverse effects on performance.

To summarise our advice so far and if practical to do so:

  • House boars and gilts separately. This can be best achieved at weaning when pigs are easier to handle.
  • Offer finishing boars (~60kg onwards) a 15% CP diet and gilts a 13% CP diet. This strategy will reduce N excretion from gilts by 18.5% while ensuring boars achieve their production performance.
  • Know the performance of your pigs - if intake rates are high consider moving all pigs in late finishing to 13% CP/ 1% lysine diet.
  • While the work supports a phased change at 60-65kg as above, there is no evidence to support a further diet change beyond this in the late finishing period.

This article can be viewed on the AFBI, Devenish and Thompsons' websites.

You can read the previous article in this series here.

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