For years, the accepted target weight for yearling replacement beef heifers at breeding was 65% of their mature weight.  Recently that target has been questioned. Oklahoma State University reproductive physiologists have studied the weight that beef heifers reach puberty in relationship to their eventual mature weight. A total of 34 crossbred heifers (in 3 different years) were studied at puberty and again when they had reached maturity at 5 to 7 years of age.  The heifers were at least ¾ Angus and ¼ or less Hereford. Shrunk weights were obtained when concentrations of progesterone in the plasma indicated that cycling activity had begun. Please remember that “average” is the point at which approximately half of the heifers reached puberty.  The “average” weights at puberty were statistically similar (695 lb, 695 lb, and 737 lb) for the three years.  Mature weights were obtained in mid-gestation, adjusted to a body condition score of 5.  Mature “average” body weights were also similar (1269 lb, 1256 lb, and 1280 lb) for cows born in all three years. The mean (or “average”) weight at which heifers reached puberty was 56% of the mature weight.  Figure 1 below shows the percentage of heifers reaching puberty at the incremental increases in percentage of mature weight.

Only 12% of the heifers reached puberty at 50% of mature weight or less.  Only forty-seven (47%) percent of the heifers reached puberty at 55% of mature weight or less.  Ninety-one (91%) percent of the heifers reached puberty at 60% of mature weight and 97% had reached puberty by the time they weighed 65% of the mature weight. Producers wanting to be certain that a high percentage (90% or more) of their replacement heifers have reached puberty before the start of the breeding season, need to have heifers weigh at least 60% of the mature weight.  For example, if a producer goes to the expense and effort of estrous synchronization and AI, getting the most heifers bred artificially is probably the goal.  In this scenario, making certain that all of the heifers weighed 60% or more of the mature weight makes sense.  Other producers may wish to place maximum selection pressure on early puberty and high reproductive soundness.  They may choose to turn bulls in with heifers at 55% of mature weight and cull any open heifers after a relatively short (45 to 60 day) breeding season. 

Source:  Davis and Wettemann. 2009 Oklahoma State University Animal Science Research Report.