If you are going to stay on your current calving period, assuming you have a defined calving period, your choices on when to breed a heifer are either 15 months or 27 months of age.  In my opinion it is not economically feasible to breed at 27 months = calving at 36 mo/3 years old.

If a heifer can't calve successfully on her own at 24 months/ 2 years old you probably don't want her in your herd anyway.

On calving period - I have been working to get as tight a calving period as possible in my small herd - 23 days/1 cycle if possible from first to last calf.

This year I had a couple of heifers I bought as bred calve about a month after my home-grown group andf there is a huge difference in the growth stage of the main group vs these two later calves. 

Here is a picture I snapped the other day of one of my homegrown 1st calf hiefers in the foreground, her April calf #3Y in the background at the mineral barrel and one of the later calving purchased heifers' calves in the background walking away. This shows the difference in the size of the calves. Haven't weighed them recently but visually a big difference.

Here also is a picture heifer #8 who was bred in June at almost exactly 15 months old. She is 17 months old now as pictured and obviously bred. Basically I would agree with TC and others that breeding a heifer is an age thing not a weight thing, assuming a reasonable heifer bull.  fwiw.

Jim at Dawn

(IMG_0976_Heifer 3 her calf and later calf 082711.jpg)

(IMG_0977_Heifer 3 her calf and later calf 082711.jpg)

(IMG_0980_heifer 8 bred in june at 15 months 082711.jpg)

IMG_0976_Heifer 3 her calf and later calf 082711.jpg (91KB - 95 downloads)

IMG_0977_Heifer 3 her calf and later calf 082711.jpg (78KB - 94 downloads)

IMG_0980_heifer 8 bred in june at 15 months 082711.jpg (86KB - 99 downloads)