The return to estrus

To increase the number of litters per sow per year, pig producers aim to inseminate the sow at the first estrus after weaning. Success is measured by a low rate of sows returning to estrus. For successful insemination, the physiological condition of the sow needs to be in a state to a) facilitate fertilization of the mature egg b) prepare the uterus for embryo implantation and c) maintain pregnancy after successful fertilization.

Sows returning to estrus result in an economic loss by reducing the number of litters per sow per year. Besides, even culling rates of sows that might not yet have achieved their optimal performance increase. A high rate of sows returning to estrus, therefore, disrupts the smooth running and the financial status of a sow farm. It is important to minimize the rate of return to estrus through good management, proper sow nutrition, insemination practice, and preparation of the sow for the next breeding cycle.

To become active, vitamin D3 needs to undergo a two-step activation. The first step happens in the liver and the second in the kidney to deliver the bioactive form 1,25-dihydroxycholecalciferol (1,25(OH)2D3). 1,25(OH)2D3 binds to the vitamin D3 receptor (VDR) to exert its action in the body. Besides the classical functions of vitamin D3 on mineral metabolism, there are also non-classical functions such as effects on fertility. It has been observed that 1,25(OH)2D3 is important for implantation and placentation. There is also evidence that 1,25(OH)2D3 plays a role in immune modulation during pregnancy and influences oocyte quality. These are factors that are important to increase chances of successful insemination and maintain pregnancy, which all play a role in reducing the number of sows returning to estrus.