Organic egg production. I: Effects of different dietary protein contents and forage material on organic egg production, nitrogen and mineral retention and total tract digestibility of nutrients of two hen genotypes

• Production parameters were affected by genotype, diet and foraging material.
• The dual purpose genotype obtained poorer production results.
• The large intake of foraging material by the hens diluted the nutrient concentration.
• Digestibility of different nutrient and AMEn was affected to a different degree.
• Retention of N and P were low and excretion of N high with increasing dietary protein.

The purpose of the study was to determine the effect of organic layer diets differing in protein and amino acid content on egg production and feed intake with two hen genotypes. Three experimental layer diets A, B and C were formulated to contain decreasing content of protein (A: 193 g/kg, B: 187 g/kg, C: 170 g/kg) but with similar energy content (average: 11.3 MJ ME/kg). No synthetic amino acids were added to the diets in line with the organic legislation, so the content of amino acids changed with the different protein content (e.g. methionine, A: 3.6 g/kg, B: 2.8 g/kg, C: 2.6 g/kg). Diets A, B and C were fed to both genotypes (Lohmann Silver (LS); New Hampshire (NH)), together with maize silage and carrots or alfalfa silage in a complete randomised design with 4 replicate pens each of 25 hens (total 1200 hens). Each pen had a separate house and outdoor hen yard of 4 m2/hen. The experiment was carried out from 18 to 41 weeks of age, and layer diets and foraging material were fed ad libitum. A balance and digestibility experiment was performed at the end of the production study. The production parameters were significantly affected by genotype, diet and foraging material. The NH genotype had an average laying rate of 70%, compared to 85% for LS, (P < 0.001). The FCR was also most efficient with LS hens fed diet A, and there was a significant effect of genotype (P = 0.001; LS < NH) and diet (P = 0.008; A < B = C) on FCR. Both genotypes showed preference for maize silage and carrots (104 g/hen/d) compared to alfalfa silage (57 g/hen/d), which reduced diet intake for both NH and LS given maize silage and carrots (P = 0.002). This decreased laying rate with diet B and C, which could indicate insufficient supply of essential amino acids. The retentions of N and P were low, however, the N content in excreta was significantly influenced by both genotype (P = 0.002; NH > LS), diet (P = 0.004; A > B = C) and forage material (P < 0.001; Alfalfa silage > maize silage plus carrots). Ca retention was significantly higher with LS hens (P < 0.001), and with diet A compared to diet B and C (P = 0.015). The coefficient of total tract digestibility (CTTAD) of the different nutrients was affected differently, having only minor effect on CTTAD of starch, being high in all groups. The CTTAD of methionine and cysteine was higher with diet A, compared with B and C (P < 0.001). It is concluded that protein and amino acid content and high intake of foraging material influence production parameters in both genotypes, where LS performed more efficiently than NH. The hens obtain most likely some nutrients from the high intake of maize silage plus carrots and alfalfa silage, however, the foraging material have diluted the total intake of nutrients to some extent, which possibly influenced the results obtained on the overall production parameters.