Magnesium (Mg) supplementation remarkably improves the digestibility of feed. In cows and sows, it has improved the reproduction and shortened the service period. In broilers it increased weight gain, and it has increased egg production of laying hens. In addition, increasing Mg intake benefits the quality of breeding eggs and improves hatching yield. However, increasing Mg intake has not altered visceral composition of embryos, although brain and liver might have the capacity to store Mg at intake above the requirement.