Nicky,
If Cidr's didn't help, then I think trailing cows wasn't the problem, nutrition was. I am going to assume you have a great mineral program and it wasn't changed, but that could be a bad one. I want to focus on some other things, but don't ignore a mineral based cause. I am assuming you never had this problem before so minerals had always been good.
How do you feed the month before turning bulls in? Calves can really pull a cow down. A good cow will send the nutrients to the calf first and then to put on condition and finally will feel like cycling. CYCLING IS A BIOLOGICAL LUXURY. This I highlight as it has been a hard learned lesson for me.
Our selection can impact this. We select for performance, calves that require more and cows that give more. This can be very subtle until one year where a change in the environment, feed value or weather, can put us over the edge. Then suddenly a larger group of cows don't cycle for awhile and this subtle, gradual change show up.
I went to OSU's deal in La Grand ein January. Select Select Sires Guy showed that many cows aren't cycling when we first turn a bull in. He had some ideas, some I am going to incorporate. I calve October and can go to a 80-90 day weaning deal here in Hermiston. the act of weaning can stimulate a cow to cycle. Main thing is she is in shape to cycle. This will work for me.
When do you calve? I note you turn the bulls in when the cows are about to be dispersed. This is why I changed my calving. I can use fewer bulls and buy better ones with the saved money. Mike Leap ranched over in your country and we talked about this a lot before. he had great luck with fall calving. Plus, calves are really the size to utilize grass when it hits.
Nicky, sorry for being so windy, you just struck the cord I have been working on for some time,