Not really sure if this observation has been made or where it belongs, but the more attuned I become to the various hints, nuances, etc. the more I seem to find. Not sure if this is totally out there but it all seems to hang together.
I always wondered why did Lureen marry Jack since she was 1) college grad, 2) had rich daddy, 3) no lack of interest from other men (ie bar scene where she is constantly being approached while waiting for Jack to do something) and 4) easy on the eye.
Also, why did LD Newsume hate Jack so much and the issue of Bobby's difficulty in school was mentioned at least a couple of times so there must be a reason.
Paying close attention to the chronology presented in the movie:
1) Jack and Lureen met at Childress County Rodeo in August 1966.
2) Jack and Ennis had reunion in October 1967 where Jack mentioned his boy was 8 months old (again no throw aways in the movie so the fact that Jack specifically stated 8 months means something)
3) Bobby was born sometime around February 1967 (Feb + 8 months = Oct)
What I can piece together based upon the few above facts:
1) Bobby was conceived in the back seat of LD's car that hot August nite (no evidence of protection, too little time to get any since Lureen had to have car back by midnite)
2) Bobby was born 2 months premature (August +7 months = February + 8 months = October), helps to explain learning disability, 2 months premature in 1967 is a big deal
3) LD hated Jack getting his darling Lureen pregnant (lets face it, Jack is a loser in LDs eyes, Jack only made $2000 the year he met Lureen and she, on the other hand, is such a great catch she could have the pick of anyone in Childress County and beyond, a lot of other people must have had the same impression of Jack as he was referred to a "pissant" by two locals)
4) Lureen didn't appear to be much of a doting mom, Jack was the one who took the time to call his teachers about his problems in school...unplanned for and unwanted pregnancy and shot gun marriage might make anyone less devoted to their child.