Is there any way that this approach does not apply selection pressure against fertility?  We want our cows calving in as tight a group as we can get because that generally will lead to a more uniform calf crop.  50 hd of 4wt calves are always going to sell better than 3 hd of 4wt calves.  Pick any weight you want, the smaller the group the poorer they will sell.  Keeping cows that can't get bred in a timely manner will hurt the fertility of your entire herd because those calves may end up back in your herd as replacements.  If your dad still calls the shots it doesn't look like you have many options.  The difficulty in addressing solely as an economic issue is that some years he may be right, those lightweight calves may still be worth having.  However, long term I think fertility of the herd is going to suffer and that may take a few years to show up.