Japan ACTORS influencing egg shell quality have been investigated by F numerous researchers. This article is presented in an attempt to share a comprehensive, but not all-inclusive, review of factors, except genetic, influencing egg shell quality. The review was done in conjunction with an investigation concerning a cracked egg problem in caged turkey breeder hens (1, 2).A review of the various techniques and devices which have been used to evaluate shell quality has not been included because of recent published reviews (3,4,). However, despite the conclusion (4) that the reported methods have not measured true shell strength, the published information on factors influencing egg shell quality is of significant value to the practical researcher because egg shell thickness (5-lo), egg shell weight (11) and egg specific gravity (12, 13) have been correlated with crack incidence.
NUTRITIONCalcium & Phosphorus: Egg shell quality of chickens whether measured by specific gravity, shell thickness, shell smoothness, breaking strength, percentage of cracks or shell appearance has been reported to be influenced by dietary calcium (14-60). Low dietary calcium levels, less than 2-0 per cent, significantly decreased egg shell quality in chickens increased egg shell thickness and egg specific gravity; although no effect has been observed in chickens (61-70) and in turkeys (2, 71, 72).The dietary calcium source (15, 73-83) and method (62, 84) of feeding calcium to chickens has been investigated. Feeding oyster .Completed while on sabbatical leave at Gifu University. Kakamigahara, Gifu,
Japan. 163improve egg shell quality. Although, zinc proteinate has been shown to increase the percentage of shell (128). Dietary manganese levels below 20 parts per million decreased chicken egg shell quality (21, 109, 129, 130); but levels in excess of 71 parts per million have not influenced chicken (109) or turkey (131) egg shell quality.Other chemical elements such as ammonium chloride (30,121, 132-134) and hydrochloric acid (1 34) have decreased the thickness and specific gravity of chicken eggs when incorporated into the diet at the 1.5 to 2.0 per cent level. Sodium bicarbonate, at the 1.0 per cent level, in the diet of chickens increased the egg specific gravity and egg shell thickness (135,136 ); although others have reported that 0.5 per cent (20, 137), 1.0 per cent (20,42, 132) and 2.0 per cent (132) was ineffective. A 0.25 per cent sodium bicarbonate level in the drinking water of laying chickens increased the shell thickness when fed a low sodium chloride diet; but no effect was observed when the diet contained 0.25 or 1.0 per cent sodium chloride (138).
Protein & Energy:Neither the dietary protein level (58, 120, 139-146), 15 to 21 per cent, protein source (42, 120, 121, 147-150), fish, animal or vegetable products nor amino acid supplementation (65, 141, 151), methionine or lysine, has been reported to influence the egg shell quality of chickens.Energy source (31, 121, 122,(152)(153)(154), corn, barley, wheat, oats, animal fat or veg...
