Phosphorus excretion through manure is an environmental issue in livestock farming, including organic egg production. New legislation in this area requires producers to reduce the amount of phosphorus spread on fields, necessitating a reduction in the phosphorus content of manure. One possible solution is to reduce the phosphorus content in feed, as this will directly lower the phosphorus content in the manure. But how far can phosphorus in the feed be reduced, and what impact does this have on productivity and egg quality? The Orpheus project has explored this in detail.

The overall aim of the ORPHEUS project has been to reduce phosphorus (P) in poultry manure through a significant reduction of P in the content of the feed. Additionally, the project aimed to develop a new strategy for calcium (Ca) allocation, which lowers the hens' requirement for P from feed, thereby reducing its excretion in manure. At AU, a trial was conducted with organic hens of the DEKALB White strain from 18 to 80 weeks of age, where feed with four P levels and two Ca strategies was tested from 30 weeks of age. The P levels varied from 0.35 - 0.30 - 0.25 - 0.20% available P, where available P is the part of the feed's P that the hen can absorb through digestion. Ca was either added to the complete feed or provided separately in a silo in the afternoon and evening. In total, the trial consisted of eight different combinations of Ca and P. Crushed mussel shells with a large particle size were used as the calcium source in all treatments. The feed with a 0.20% available P content was produced without the addition of inorganic P, meaning the P came solely from the raw materials used in the feed.

The effect of reduced phosphorus has been investigated on a wide range of production parameters in this trial – including the impact of reduced P in feed on eggshell quality. For example, it is important that hens produce the same number of eggs as usual, that the egg size is as expected, and, most importantly, that the eggshell does not deteriorate or weaken. In the trial, Danæg measured shell strength at the various combinations of Ca and P and at increasing hen ages to monitor whether the different levels of Ca and P had a negative effect on eggshell strength. It was crucial to investigate the effect of the feed’s P content on eggshell quality because a poor eggshell results in more cracked eggs, i.e., eggs that get damaged during the collection and packaging process. Cracked eggs lead to losses for both the producer and the packing facility, as well as resource waste in the form of eggs that must be discarded.

Ten series of shell strength measurements were performed on eggs from the eight different combinations of Ca and P. The first measurement was taken at 24 weeks of age as a kind of "baseline test" to provide an overview of the general shell strength level. This measurement was carried out approximately six weeks before the hens were switched to the trial feed mixtures. The second measurement was taken when the hens were 34 weeks old and had been on the trial feed for about four weeks. Subsequent measurements were taken periodically during the production period. The average shell strength decreased with increasing hen age, which was expected. However, the shell strength in the trial was generally better than expected for DEKALB White at a given age. This was true for all trial combinations and at all ages.

During the shell strength measurements, the eggs were also visually assessed for the frequency of porous or thin-shelled eggs. In all measurements – from 24 weeks until the end at 80 weeks – the eggs were visually uniform and consistent in shell quality. Few or no eggs were porous or visibly thin-shelled.

The results of the shell strength measurements are shown in Figure 1, where data is summarised by feeding strategy, i.e., for the four levels of P with Ca added to the feed (complete feed) and the four levels of P with Ca provided separately (mussel shells). In the figure, the current recommendations for shell strength are also included for comparison, and it shows that shell strength was good across the various combinations, especially for hens that were provided mussel shells separately. Overall, throughout the trial, groups with separate provision of mussel shells had better shell strength compared to those that received all Ca in the feed. With separate provision of mussel shells, the hens have a better opportunity to adjust their Ca intake in relation to the P content in the feed. The results show that with the option to adjust Ca intake, a reduced P content down to 0.2% available P had no negative effect on shell strength.

During the formation of the eggshell, 2-3g of Ca is deposited per egg per day. This amount of Ca must be mobilised by the hen for daily eggshell production, which spans 18-20 hours. The expected amount of deposited Ca remains relatively constant throughout the egg production cycle, and as egg weight increases with age, one might expect this to be the cause of the observed decline in shell quality. To investigate whether shell strength/quality declined with increasing egg weight, egg weight was recorded for each egg during the shell strength test. Surprisingly, no correlation was found between egg weight and shell strength in this trial, as illustrated in Figure 2 below. Additionally, Figure 3 shows that egg weight remains reasonably stable from 45-50 weeks of age, which is an important characteristic when hens have a long production period and is typical of the DEKALB White hen.

Our conclusion on declining shell quality with increasing hen age is therefore that shell strength generally decreases with increasing hen age and not because of increasing egg weight. There is some evidence that the amount of calcium required for eggshell formation is not the same whether the egg weight is low or high, but that the amount required for shell formation is more dependent on hen age. It would be interesting to investigate the weight of the eggshell at different egg weights and hen ages to determine whether this theory can be confirmed.

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This article is an extended version of an article published in the Økologisk Landbrug newspaper, May 2023.

The ORPHEUS project is part of the Organic RDD 5 programme, coordinated by ICROFS (International Centre for Research in Organic Farming and Food Systems) in collaboration with the Green Development and Demonstration Programme (GUDP) under the Ministry of Food, Agriculture and Fisheries.