Consumers link egg quality to yolk colour. Depending on the nutrition of the laying hen, yolk colour can vary from a very pale yellow to deep orange. Pigments are transferred directly from the diet to the forming egg yolks in the ovaries. Hens fed diets based on wheat, barley, sorghum or other non-pigmented grains will produce eggs with pale yolks. Eggs from birds fed a corn-based diet will have yellow yolks, while those from hens fed natural or synthetic pigments, or such feed ingredients as alfalfa meal, will be various shades of orange. Yolk colour is scored on a 1 to 15 scale with 1 being very pale yellow and 15 a deep orange.
Yolk colour is influenced by many factors including: ingredients used, pigment (oxycarotenoid) levels both in ingredients and additives, length of storage, feed intake of birds, stability of fats in the feed, antioxidants, calcium, vitamin A, drug use, mycotoxins, health, hours of daylight.
The oxycarotenoid content in feed ingredients can vary. Corn and corn products contains higher oxycarotenoids compared to that of wheat and barley. Therefore, layers fed with corn as the main grain will produce a yolk colour score of 9 on the 1 to 15 scale. However layer feed with wheat or barley will only produce yolk colour with score of 5 or less. Good green pasture such as lucerne contains high oxycarotenoid levels and when used with a high corn diet layers can produce eggs with a yolk colour score of 10.
Oxycarotenoids are fat related material, so they are very sensitive to oxidation. Therefore, when feed is stored in hot conditions for a long period of time it will start to lose its pigment value. Poor quality oil/fats in the feed will increase the risk of the oxidation and extra antioxidant such as vitamin E can be added into feed will slow down oxidation.
High levels of calcium or vitamin A in the feed may also reduce the yolk colour. Mycotoxins presents in the feed will reduce the absorption of the pigment from the gut, affect the transport of the pigment to the ovary and reduce the deposition of colour in the yolk.
Daylight can increase the pigmentation of yolk. Light may change the yellow pigment to orange if the feed is exposed to light and may change the pigment metabolism of the animal. Studies have shown that birds exposed to good daylight can produce egg yolks with extra yolk colour score compared to birds in cages.
Any event that reduces feed intake such as hot weather, water shortages, no feed will also reduce yolk colour score.
If you are experiencing problems with yolk colour contact your local Riverina representative or nutritionist.