I have read a lot of research and everything I have seen is that feeding pregnant cows and heifers helps them breed back and calves are healthier. Cutting a cow or heifer off from feed for 3 months or more is extreme. Likewise, if you pen up a heifer like she is in a feedlot and have feed free choice it is extreme. If you feed a cow 1 percent of her body weight and you have calving problems then I would argue that there is a problem with the animal not with the feed. To me if you are trying to breed a heifer to calve at 2 years old, which is a common practice, your heifer will never reach her full potential without feed. The caveat to that is if you have the ideal forage so that you have quality grass and your cows have body condition then that's a different st
ory. Even with that you would likely have to supplement with protein and minerals. By the way from what I have seen with recips the only time that weight hurts fertility is if they are obese...Body Score 9.
This is from University of Nebraska-Lincoln
Thin cows at calving (BCS 4 or thinner) produce less colostrum, give birth to less vigorous calves that are slower to stand and these calves have lower immunoglobulin levels (Table 2, below), thus impairing their ability to overcome early calf-hood disease challenges. This illustrates the importance of targeting mature cows to calve in a BCS of at least 5. Because 1st-calf-heifers have only reached about 85% of their mature weight after calving and require additional nutrients to support growth, they need to be fed so they are a BCS of 6 at calving.