Lut will only work if they are currently cycling and within a certain time frame in that cycle (days 6 thru 14??). Anyway, if they are naturally cycling, the bull will get them bred as they "naturally" come in. 60 days is MORE THAN ADEQUATE time for a bull to get them ALL bred.
16 months at 800# sound a little light to me, but even at that, if they are receiving good nutrition, mineral & water - they should be cycling.
One shot to 1/2 the heifers will make only a portion of them come into heat - only the ones that are within that time frame.
The proper way to use Lut is :
1. "Know" when they cycled & give shot 6 days after.
2. Breed for 5 days, give a shot to those that you did not catch in heat, the rest should cycle.
3. Give 1 shot - those in the right time frame will cycle about 2-3 days later, which you can breed those cycled heifers - than, 11 days after first shot, you give a shot to those that did not cycle & get bred.
4. Give 1 shot - don't breed anything - give 2nd shot 11 days later - all should be in heat 2-3 days later. All of these options work IF, AND ONLY IF they are already cycling.
Also, if you have a bull running with them, you won't REALLY know if they got bred by the bull, so you don't want to give anything exposed to the bull a shot of Lut, because Lut also ABORTS pregnancy.
So, sounds like to me you're only option with the bull coming this weekend, is give them all Lut shot before they go out with bull - which will kill the bull :lol:
or, leave them alone. Like I said 60 days should be more than enough time. If they are not bred in that time frame - they are poor quality - poorly raised - or poor breeders and should be culled.