The difference between 21-day weaning and 28-day weaning is mainly due to the demand for sow milk production and the growth and development of piglets. In the case of 21 days of weaning, the sow's milk production decreases rapidly after weaning, which helps to stimulate the sow to come into heat faster and get pregnant again. However, this practice may affect the growth and development of piglets as they need to adapt to new feed and water after weaning, which may lead to diarrhea and other digestive problems in piglets.
In contrast, 28 days weaning can provide longer breastfeeding and contribute to the development of the piglet's immune system and digestive system. In addition, this also allows the sow more time to recover after weaning and prepare for the next pregnancy. However, weaning at 28 days may increase the cost of feeding sows as they need to consume more feed to produce more milk.
Overall, the choice of 21 or 28 days of weaning depends on the specific circumstances of the farm, including the milk yield of the sow, the growth and development needs of the piglets, and the economic benefits of the farm. In practice, many pig farmers use 21-day weaning, but this is also adjusted on a case-by-case basis. It is important for piglets to be provided with proper feed, drinking water and care to help them make a smooth transition to independent living.