Extension.org reports a common cause of anestrus, also known as non-cyling, in sows. This issue is especially important in sows weaning their first litter. 

Producers should evaluage feeding frequency, feeder and waterers design, and the nutrients in feed. There may also be a need to add energy or increase the protein in the lactation diet when feed intake is load. Furthermore, imporper feeding could lead to excessive weight loss during lacation or insufficient weight gain during pregnancy. Both are primary considerations when anestrus occurs following weaning. 

"The stress of grouping sows and withholding feed after weaning will lengthen the average interval to estrus," the report said. "Thin sows should be fed 6 to 8 pounds of the gestation diet for at least the first week after weaning. Housing sows in crates or small groups may increase the percentage of sows that cycle early. And, of course, (you have already done this), exposure to a mature boar, either in adjacent pens or by daily movement of the boar among the sows, should stimulate cycling. Mixing 3 or 4 sows with the boar for the first 48 hours after weaning reduces fighting among sows and provides boar stimulus to initiate cycles." 

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