Martin Smith Thanks for your comments. This is a great input and let me explain more. In practical diets for broiler breeders the methionine (M) and cysteine (C) are limited and most diets cover the minimum requirements. And flocks fed with levels of M+C that match the requirements shows egg oversize. Why? In practical diets of broiler breeders the digestible lysine is above the requirements, meaning the synthetic lysine is almost not used in breeders diets, except in the pre-starter pullet diets, said this, the excess of lysine coming from the ingredients produces egg oversize and big muscle yield. Sure, in practical diets other amino acids along with lysine such as isoleucine and leucine for example are also at high levels when lysine is above the breeder requirements using corn and soybean meal. On the other hand, I did a review with 13 review-papers that show the amino acids requirements using synthetic amino acids in broiler breeders. In this analysis, the most important amino acids that reduced the egg size was the M+C (1.1 g), lysine (1.2 g), and valine (-1.6g). However, they also reduced the egg production, first was the M+C (-6.6%), then Lysine (-4.0) and then Valine (-2.0). This is reducing at 25% less of the AA requirements. I did lab and field studies, and they indicate that the breast meat and egg size correlate strongly. See the studies of Dr. Coon;s lab that I participated on them, specifically Ekmay's paper. As broiler breeder hens age, the lysine from the eggs coming from lysine of the muscle increases; we did this study using labeled amino acids. Further, working in the field with broiler breeders, I used a device to measure the angle of breast meat in vivo. In flocks with more angle meaning more breast meat, the hens layed bigger egg size. Further, when we touched the breast meat of hens from egg oversize, the flesh was harder, like a rock. In contrast, in hens from smaller eggs, the breast meat was softer by palpation. After this observation, I measured by a durometer, the harness of the breast meat in live hens, and indeed they reported higher numbers, meaning hens with egg overweight also had harder breast meat. With clients I see the same output: when the lysine is reduced to match the breeder requirements, the egg size was reduced and hatchability improved. I agree with you also that egg size is also determined by the genetic. There are lines that tend to produce a larger egg size than other. Further, another important point is the level of egg production, flocks with poor egg production tend to produce larger egg size. Hope this helps in the discussion. And best regards.