What could cause anestrus or non-cycling sows?

A common cause of anestrus in weaned sows is insufficient energy or protein intake during lactation. This is particularly important in sows weaning their first litters. The frequency of feeding, the design of the feeders (large enough for sows) and waterers, and the nutrients in the feed should be evaluated. There may be a need to add energy or increase the protein in the lactation diet when feed intake is low. Improper feeding which leads to excessive weight loss during lactation or insufficient weight gain during pregnancy are the primary considerations when anestrus occurs following weaning.
Conversely, excessive feed intake during gestation (overweight sows) will lead to decreased feed intake during lactation resulting in severe weight loss and sometimes anestrus after weaning. The length of lactation also influences return to heat. Sows with short lactation, particurlarly if less than 21 days, may require more time to cycle after weaning. There is evidence to suggest that weaning the heaviest pigs in the litter at least 48 hours early will improve cycling performance, especially in first-litter sows.
The stress of grouping sows and withholding feed after weaning will lengthen the average interval to estrus. 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.
The diagnosis of anestrous problems should be based upon heat detection and breeding records. Eight percent of the gilts should be cycling regularly by 8 months of age, or an average of 4% of the gilts of that age should have cycled within the first 10 days following weaning. Estrous detection methods and frequencies should be checked closely to assure proper procedures are followed.
Other diagnostic procedures might include slaughter examinations or obtaining serum progesterone levels to determine if estrus has been missed. Also, feed that the sows are consuming might contain some mycotoxin, such as aflatoxin or zearalonone.