I would suggest that you get an analysis of your brome hay, and determine if that is sufficient for the requirements of heifers, and then balance the hay with what is necessary to supplement it. Make sure that you are feeding a balanced mineral - everyday!
High heat and endophyte-infected fescue don't affect pregnancy rates of adult beef cows, but a study shows that heifers that are heat-stressed and consume toxic fescue are less likely to produce calves. That may be a significant possibility of your difficulty in getting the second pregnancy to take. Another factor which may be a causitive element in your difficulty in re-breeding first-calf heifers is their body condition at the time of re-breeding. Here is a link that may be of some help to you.
http://wwwclemson.edu/edisto/beef-db/bc-2005.htm
DOC HARRIS