We have had our own chickens for over 20 years and feed them on crushed grain and laying pellets and they eat greens in the fields. The fresh eggs have always been plump in the centre when cracked. Lately the yolks are flat and even if carefully broken into the pan they tend to rupture and leak as the egg is frying. Just like old eggs. what could be the explanation?
Age, pure and simple. I used to be a free range egg producer and our chickens were retired to homes happy to have them as pets and occasional layers by the time they were two years of age.
Eggs become larger as they age but as they get beyond two, most hens lay a lot less and quite frequently the eggs can show their age on the inside as well as outside.
Like humans, hens have only so many eggs they are born with and once those have all been released everything starts to slow right down until it stops completely.
The information below came from eggfarmers.org.nz WARNING, This link is a PDF file. (nz stands for New Zealand, but there is some good insight available on this .pdf).
Yolk quality is determined by the colour, texture, firmness and smell of the yolk (Jacob et al., 2000).
Although yolk colour is a key factor in any consumer survey relating to egg quality (Jacob et al., 2000), consumer preferences for yolk colour are highly subjective and vary widely from country to country. In general, New Zealand consumers prefer a yolk colour between 11 and 13 on the Roche (now DSM) Yolk Colour Fan (Sutherland, personal communication).
The primary determinant of yolk colour is the xanthophyll (plant pigment) content of the diet consumed. It is possible to manipulate the yolk colour of eggs by the addition of natural or synthetic xanthophylls to layer hen feeds. This ability to readily manipulate egg yolk colour can be an advantage in meeting market demands.
However, the ease with which yolk colour can be manipulated can lead to unwanted colour changes. For example, the inclusion of higher than recommended levels or incorrect ratios of pigments can lead to orange-red yolks (Coutts and Wilson, 1990). Similarly, diphenyl-para-phenylenediamine (DPPD), an antioxidant, has been reported to cause excessive deposition of pigments in the egg yolk (Coutts and Wilson, 1990).
The inclusion of more than 5 % cottonseed meal in a layer diet will result in olive or salmon coloured yolks (Beyer, 2005), while the inclusion of certain weeds or weed seeds may results in green yolks (Beyer, 2005; Coutts and Wilson, 1990). Similarly, inadvertent omission of xanthophylls from the diet will lead to pale yolks. Both inadequate mixing of the diet as as well as excessive mixing of the diet will also result in a heterogeneous feed, and subsequent variation in the amount of xanthophylls consumed by each hen in the flock, This will result in egg yolk colour not being uniform throughout the flock.
Pale yolks can result from any factor which alters or prevents the absorption of pigments from the diet or the deposition of these pigments in the yolk. These factors could include;
- worms (Coutts and Wilson, 1990)
- any factor which inhibits liver function, subsequent lipids metabolism and deposition of pigment in the yolk. For example, mycotoxicosis caused by aflatoxin B1 (Zaghini et al., 2005).
- coccidiosis, although this is rare in adult hens.
Mottled yolks: (with many pale spots and blotches which vary in colour size and shape), occur when the contents of the albumen and yolk mix as a result of degeneration and increase permeability of the vitelline membrane (Jacob et al., 2000). Factors affecting mottling were reviewed in detail by Cunningham and Sanford (1974).Dietary factors which may cause mottled yolks include;
- The presence of nicarbazin (an anticoccidal agent) in the feed has shown by numerous authors to cause mottling (Jones et al., 1990; Cunningham and Sanford, 1974)
- Worming drugs such as phenothiazine (Coutts and Wilson, 1990), dibutyltin dialaurate (Jacob et al., 2000; Coutts and Wilson, 1990; Berry et al., 1968, cited by Cunningham and Sanford, 1974) and Piperazine (Jacob et al., 2000; Coutts and Wilson, 1990). However, Berry et al. (1968, cited by Cunningham and Sanford, 1974) did not observe yolk defects when Piperazine was fed at the manufacturer’s recommendations. Similarly, they only observed defects when dibutyltin dialaurate was fed at the recommended level but over a much longer period.
- Gossypol from cotton seed meal (Jacob et al., 2000; Berry et al., 1968, cited by Cunningham and Sanford, 1974)
- Certain antioxidants such as gallic acid (from grapes, tea and oak bark) and tannic acid (Coutts and Wilson, 1990), or tannins from grains such as sorghum (Jacob et al., 2000)
- Calcium deficient diets (Jacob et al., 2000; McCready et al., 1972, cited by Cunningham and Sanford, 1974)
Storage time and temperature has also been shown to affect the degree of egg yolk mottling (Jacob et al., 2000; Coutts and Wilson, 1990). Jones (2006) stated that as the internal temperature of the egg increases above 7 degrees Celsius, the protein structures of the thick albumen and vitelline membrane breakdown faster. As the membrane degenerates during storage, water enters the yolk causing mottling and after prolonged storage, albumen proteins also enter the yolk increasing the severity of mottling (Jacob et al., 2000). In order to reduce the rate of breakdown of the vitelline membrane, eggs should be collected regularly, reducing the time they are exposed to higher environmental temperatures and contaminants, and stored at temperatures of 7 - 13 degrees Celsius and humidity of 50 - 60 %.
In their review, Cunningham and Sanford (1974) also identified hen age, oil coating of eggs and movement of eggs as possible factors affecting mottling of eggs.
The yolk of a freshly laid egg is round and firm (Jacob et al., 2000). However, as the egg ages and the vitelline membrane degenerates, water from the albumen moves into the yolk and gives the yolk a flattened shape. Yolk texture: Rubbery yolks may be caused by severe chilling or freezing of intact eggs, the consumption of crude cottonseed oil or the seeds of some weeds (Jacob et al., 2000)