Think of the millions — if not billions — of dollars being spent to fund research, and you probably think of the funding behind cancer research or research focusing on other human-health problems.

Millions of dollars also have been channeled into heifer research -— specifically, into studies that seek to understand the link between fast growth in heifers before puberty, mammary development and first-lactation milk yield.

Use this question-and-answer format to learn what the research has to say about rapid growth rates between three and 10 months of age, and the impact it has on mammary gland development and future milk production.

Q. Does a faster rate of gain impair mammary development?

Most experts agree that growing heifers too fast between three months of age and puberty (eight to 10 months) hurts mammary development — and ultimately, first-lactation milk yield.

This time period is critical to mammary development. In fact, the mammary gland grows more rapidly during this time than after puberty, says Mike VandeHaar, professor of animal science at MichiganStateUniversity.

“There is a level that’s too fast,” VandeHaar says. However, the experts don’t all agree on what that level is.

The data show that gains greater than about 2 pounds per day for Holsteins cause impaired mammary development, VandeHaar says.

However, a recent analysis of eight studies on this topic begs to differ. That analysis, published in the November 2005 Journal of Dairy Science, suggests the cap may be 1.8 pounds per day. (For more information, see “Set limits on gains before puberty” on page 37 in the January issue.)

Bottom line: Growing heifers too fast before puberty — in excess of 1.8 to 2.1 pounds per day — hurts the development of the mammary gland.

Q. Does impaired mammary development hurt future milk yield?

During the past 20 years, researchers accepted the idea that in heifers fed for rapid gains before puberty, impaired mammary development and reduced milk production went hand-in-hand. Lately, though, that link has come under scrutiny. And while researchers still think there may be a connection, it’s not as clear-cut as once thought.

“We don’t have the best data to show they’re connected,” VandeHaar says.

First of all, in some studies, researchers slaughtered heifers before their first lactation. By doing so, they were able to gauge mammary development, and whether or not a faster rate of gain hurt development. But, it’s unknown how these heifers would have performed during their first lactation.

Second, the research that has followed heifers through their first lactation shows conflicting results. Some studies show very little impact on milk production — what researchers would say is not “statistically significant.” Others show at least a 10-percent drop in first-lactation milk yield in heifers fed for gains greater than 2 pounds per day before puberty. (For more details, please see “The research speaks for itself” below.)

The answer grows more complicated. Some researchers also suspect it may have little to do with the mammary gland, and more to do with heifers getting too fat.

When you feed heifers a higher-energy diet so that they gain more rapidly, they reach puberty earlier, explains Mike Van Amburgh, associate professor of dairy nutrition at CornellUniversity. At the same time, these heifers pack on more fat — an extra 60 to 70 pounds by the time they reach puberty (and even more by calving). Fat heifers calve with greater difficulty, have less dry matter intake capacity and achieve lower peak milk production, he says.

However, until more research is done, the exact mechanism for reduced milk production in heifers grown fast isn’t known yet.

Q. How fast can I safely grow heifers before puberty?

Based on the research, it’s probably best not to exceed 2.2 pounds per day for Holsteins. Some would argue that 1.8 pounds per day is the upper limit.

Keep these thresholds in mind, but also base your decision on how fast to grow heifers on the dynamics of your own herd. In other words, the “gold standard” of calving Holstein heifers at 24 months of age and weighing 1,250 pounds isn’t for everyone.

Instead, set goals based on the expected mature bodyweight of cows in your herd. Van Amburgh’s advice is to calve heifers at 83 percent to 85 percent of their expected mature bodyweight.

Set breeding goals based on expected mature bodyweight, too. For example, a 1,600-pound mature weight Holstein should be pregnant by 55 percent of her mature bodyweight, or 880 pounds, and achieve a post-calving bodyweight of 1,360 pounds, he says.

Q. How do I know if my heifers are growing too fast?

Some heifers grow fast without gaining a lot of fat, VandeHaar says. Others pack on too much fat by puberty. How do you know if they’re on track?

Weigh heifers on a scale or use weigh tape at weaning and at breeding — at the very least, VandeHaar says. Monitor body condition, too. Slow things down if gains exceed 2 pounds per day and body condition scores are greater than 3.5 on a 5-point scale.

It’s OK to grow heifers fast — up to a certain point, before it becomes too risky for mammary development and future milk yield. Think about what you need to do in your calf- and heifer-rearing programs to achieve your age-at-first-calving goals, without sacrificing these objectives.