Utility of nettle (Urtica dioica) in layer diets as a natural yellow colorant for egg yolk

Yolk color is an important quality trait of eggs. Natural pigment sources are preferred by consumers. Synthetic pigments are banned in some production systems. As nettle (Urtica dioica) addition was found to substantially increase broiler skin yellowness, it was hypothesized to be a potent natural yolk coloring feed component. Therefore, the pigmentation by nettle and possible side-effects on performance, egg quality and antioxidant properties were tested in a 4-week experiment with 40 individually caged H&N Nick Brown layers (70 weeks of age). A basal feed mixture low in pigments and tocopherol was designed. Two weeks prior to the experiment, all animals received this basal mixture plus synthetic pigments (25 mg/kg Carophyll (R) Yellow, 15 mg/kg Carophyll (R) Red). Thereafter, eight animals each received diets either with 0, 6.25, 12.5 or 25 g nettle per kg put into the basal mixture or a control diet containing synthetic pigments like the pre-experimental diet and extra 40 mg/kg of alpha-tocopherylacetate. In detail, nettle was supplied by two independent batches to four animals per dosage each. Performance was assessed during 4 weeks and feed and egg samples were collected in the last week of the experiment and analyzed for various variables with a focus on color and antioxidant traits. By applying different statistical models, a comparison with the control animals, batch differences and nettle dosage effects were evaluated. Yolk yellowness (b*) increased with nettle addition depending on dosage and batch, yet was equally effective as synthetic pigmentation (29.4) in all investigated cases (avg. 30.3). The score according to the DSM-Yolk Color Fan increased from 1.7 in the non-supplemented group to 4.2 (6.25 g nettle A/kg) and up to 6.5(25 g nettle B/kg). This increase depended on dose and batch. Due to the lack of red pigments in nettle, scores were still lower than with the control treatment (9.5). The development of thiobarbituric acid reactive substances, monitored over 12 weeks in lyophilized yolk powder, was not affected by batch or nettle concentration. However, yolk from nettle fed hens, especially from one batch, was richer in tocopherols with increasing dosage though being far from that found in the tocopherol supplemented hens. There was no substantial influence of nettle supplement or batch on laying performance and general egg quality. Nettle supplementation of layer diets is therefore considered as an effective means to naturally achieve the desired yolk yellowness, and this without risking unfavorable side-effects. (C) 2013 Elsevier B.V. All rights reserved.