A little phosphorus diet? No problem, our chickens adapt!

Authors: Anne-Sophie Valable, Piterson Floradin, Laeticia Cloutier and Marie-Pierre Létourneau Montminy

Article published in Nouvailes, September 2023, #25, Pages 47 to 49

To reduce the environmental impact of our farms, it is important to feed our chickens precisely, that is to say by limiting excess nutrients, including phosphorus (P). Knowing that our chickens use little phosphorus from the grains consumed, phosphorus must be added to the feed, the latter coming partly or entirely from phosphate rock, a limited non-renewable resource. Indeed, in the European context in particular and in all-vegetable feeds, animal meal, a sustainable source of P, is not permitted. Phosphorus can therefore generate not only significant costs, but when provided in excess in the feed, it constitutes a major pollutant of aquatic ecosystems. In a context of sustainable production, it is necessary to find feeding strategies to control phosphorus inputs and limit its excretion into the environment, while maintaining optimal growth performance in animals.

A small phosphorus diet…

Several studies carried out in poultry nutrition in recent years have shown that it is possible to reduce phosphorus and calcium intakes by 20% to 30% for the entire flock, without affecting growth performance, by using a “depletion-repletion” strategy. This is due to phosphocalcic regulations that aim to maintain stable blood levels, which is vital.

Previous studies have shown that the use of this strategy in chickens has the potential to increase phosphorus absorption by 15–35% and reduce rejection by 40–60%. However, its effect on the recovery of bone losses is not clearly observed in all studies, with some observing complete recovery of bone deficit at the end of the repletion phase and others not. It would appear that the ability of animals to compensate for bone mineralization losses depends on several factors such as age, dietary phosphorus and calcium intake levels, the timing of depletion or repletion, and the type of bone analyzed.

What is a “depletion-repletion” strategy?

This consists of a phase of reducing nutritional intakes of phosphorus and calcium, aimed at triggering physiological adaptation mechanisms to increase the use of phosphorus and calcium, known as “depletion”, by increasing absorption for example. This is followed by a compensation phase where phosphorus intakes coincide or exceed needs to allow the recovery of losses caused by depletion, known as a “repletion” phase.

Why do we always talk about the need for phosphorus and calcium?

There are different links between these two minerals. At the bone level, they are physically linked in the form of hydroxyapatite, the deposition of one depends on a joint supply of the other otherwise the excesses are excreted in the urine. At the intestinal level, they can form insoluble Ca-phosphate and Ca-phytate complexes limiting the use of the two minerals by the animal. Finally, at the level of their homeostasis, a mechanism which aims to maintain fixed concentrations necessary for the proper functioning of the body, they are regulated by the same hormones. Thus, an imbalance in one of the two minerals will influence the use of the other.

Planning the depletion-repletion strategy

In order to plan an optimal depletion-repletion strategy ensuring both the maintenance of bone mineralization at the slaughter of chickens and the reduction of phosphorus rejections, the work carried out by Anne-Sophie Valable, as part of a joint thesis supervision between Laval University in Canada and the University of Tours in France, tested the effect of strategies for reducing phosphorus and calcium intake on bone recovery in chickens (Figure 1). Bone mineral status in these trials was estimated by conventional methods and by determining bone mineral content using a dual-energy X-ray bone densitometer (DXA). The efficiency of digestive phosphorus use was assessed by measuring ileal digestibility and expression of intestinal transporters.

Figure 1: Depletion and repletion strategies tested*

Different depletion and repletion strategies were thus compared. For the depletion period, the first strategy consisted of reducing phosphorus and calcium intakes during the first 8 days of rearing (LLHL, Figure 1 yellow), the other two strategies consisted of shorter depletion periods either during the first 4 days only (LHHL, Figure 1 ochre) or from the 4th day to the 8th day (HLHL, Figure 1 brown). Then, all the strategies offered a repletion period from days 9 to 18 (H) then some ended with a lower period during days 19 to 33 (LLHL, LHHL and HLHL). All of these strategies were compared to a standard intake strategy (HHHH). The depletion periods represented decreases in phosphorus intakes compared to current recommendations varying in the order of 20 to 40% for the start-up (days 0 to 8) and finishing (days 19 to 33) phases.

Results…

The different depletion-repletion strategies tested in the trial did not modify growth performance, but significantly decreased bone mineralization in the starter and growth phases. However, in the finishing phase, following repletions, the bone mineral content of the tibia was similar for all treatments (Figure 2). These results suggest that animals were able to catch up on their bone mineralization deficit despite periods of depletion.

Figure 2. Effect of phosphorus and calcium depletion-repletion on tibia ash content at day 4 (H vs. L diets from 0 to 4 days), day 8 (HH and LH vs. HL and LL diets from 4 to 8 days), day 18 (HHH, LHH, HLH, and LLH diets from 8 to 18 days), and day 33.*

Regarding the timing of depletion, it should be noted that when applied late (between 10 and 21 days; HLHL), it does not allow metabolic adaptation compared to its application in the starter phase. Indeed, the results showed that with an initial depletion carried out from 0 to 8 days, the animals could adapt to a low phosphorus diet. A subsequent reduction in intake during the finishing phase (LLHL) did not affect bone mineral status at slaughter and allowed a maximum reduction in P discharges.

Conclusion

Our results show that the depletion-repletion strategy represents an effective alternative feeding strategy to improve the sustainability of broiler farms, especially when they depend on mineral phosphates. This will be validated using microbial phytase and by monitoring the evolution of bone mineralization in the same bird, which is now possible within the research team of Marie-Pierre Létourneau Montminy at Université Laval.