You find a brass lamp. Rub the lamp. Out pops a genie who can make your wishes come true.

It happened to a rancher one day (in my dreams). He asked the genie which of his heifers would make the best replacements. The genie berated the rancher for asking such a tough question. At the genie's insistence, the rancher went on to a second wish and asked to know the innermost thoughts of geneticists and why they exist. To which the genie replied, "Would you like to know only the heights of those heifers or their probable mature weight, too?"

Finding a genie-filled lamp would be nice when selecting heifer replacements but you can be your own genie in foreseeing some things about their futures.

One of the rules in replacement heifer development is to achieve 65 percent of mature body weight by breeding time. Mature body weight? What's that? Only a genie would know what that's going to be. But genies only exist in fables.

I recently did an analysis of 15 replacement heifers. I compared their individual breeding weights with 1,100-pound mature cows, which is the target for their herd. The results were not acceptable. The group's average was 72 percent and the individual percentages ranged from 84 percent to 59 percent. I decided to try a different approach.

In dealing with the cow-size question in this column and producer workshops in the past, I used these rules: Frame five cows with condition scores of five to six weigh 1,200 pounds and weight varies 100 pounds between frame scores. Since the frame scores of the heifers were known, this made it theoretically possible to project the mature weights and breeding-weight percentages of each.

This method produced results that were more useful. The group average fell from 72 percent to 68 percent and the individual percentages changed to a range of 81 percent to 60 percent. These changes may not look all that great on the average but there is more than meets the eye. The large-framed and small-framed heifers changed a lot. Two heifers in my test with identical frame scores of 6.25 dropped 10 and 11 percentage points; one small-framed heifer increased eight points.

It would take a genie, too, to foresee whether a frame score calculated at 14 months of age, as was the case with the heifers in this sample, would remain constant to maturity. I asked this question of John Crouch, director of performance programs at the American Angus Association. Mr. Crouch referred me to tables on mature cow size in the Angus association's Spring 1999 Sire Evaluation Report. The answer: Yes, on the average.

The Beef Improvement Federation's latest frame score tables permit calculations on females up to 48 months of age. Cows in the Angus Association's data base that were from two to four years of age varied only a half frame score. But, although this variance is based on average hip heights, it probably applies adequately to the average in your herd or the individual heights of your heifer replacement candidates.

It excites me that there are ways of fulfilling wishes in animal breeding by using existing information. Expected progeny differences for mature size already exist for some breeds, and these EPDs can take some of the guesswork out of replacement heifer selection. But there's still this thing called genetic variation that accounts for a frame score range in your better heifers when you must decide whether they're to be shipped or saved.

The little system I have used probably lacks precision, and you may already have devised something better. Either way, using existing information will have to do until someone trips over a brass lamp and convinces a genie to help with the process.

To contact Fred Knop, write Drovers or send e-mail to: fredlyn@aol.com