The most important criteria for breeding a heifer is not age but rather size (and weight). Different breeds will have different target weights.
"Generally, a heifer should be at 60 per cent of her mature body weight when first breeding occurs. The recommended weight for a breeding age Holstein heifer is 395 kilograms (870 pounds), and the recommended height 127 centimetres (50 inches). Subpar nutrition and disease in the first few months of her life will greatly affect whether she meets these growth targets."
http://www.omafra.gov.on.ca/english/liv ... heifer.htm
http://beefmagazine.com/blog/what-s-bes ... ef-heifers
Why is 2 yrs the ideal age at first calving?
"Age at First Calving
Producers should aim to calve their replacement heifers by two years of age. This practice will improve the profitability of the beef operation by offsetting the high costs of feed, labour and investment in raising replacements. To produce the maximum pounds of calves in her lifetime, a cow must calve each year starting as a two-year-old. Research from Oklahoma shows that this is indeed true-heifers calving at two years of age produced an extra 330 pounds of calf on a lifetime basis over heifers calving at three years of age."
http://www.omafra.gov.on.ca/english/liv ... 67.htm#Age
"Nutrition and Development of Replacement Heifers
Optimum reproductive performance and lifetime productivity of a cow are clearly tied to proper nutritional management of replacement heifers during growth and development of structure and reproductive function.For heifers to reach puberty at 14-15 months of age, they must be adequately grown, but not overconditioned. Three factors associated with puberty in the replacement heifer are weight, age and breed. Weight is thought to be the major determining factor affecting puberty in heifers at 14-15 months of age. Larger, later maturing breeds reach puberty at an older age.Producers, who have the most control over weight gain in their cattle, should establish target weights at which heifers are to be bred and develop a feeding program to allow heifers to reach that goal in good condition. Target weight will vary with each breed."
Please refer to:
"Table 1. Weight at Which 14-15 Month Old Heifers Reach Puberty.
Replacement beef heifers should attain 65 to 70% of their potential mature weight by the time they are bred at 14-15 months of age. This would mean that heifers should gain an average of 1.25 to 1.75 lb. per day from weaning to first breeding or 250 to 350 lbs. during the first winter (depending on breed). For most breeds and crosses, heifers should weigh from 650 to 850 lbs. at breeding time."
http://www.omafra.gov.on.ca/english/liv ... #Nutrition