Feeding heifers for a faster rate of gain before uberty may not have as big of an impact on mammary development as once thought, according to research from CornellUniversity. The findings also throw cold water on the long-held theory that impaired mammary development hurts subsequent milk yield.

Does a link exist?
During the past 20 years, many research studies have sought to establish a link between rate of gain, mammary development and future milk yield. Several of those studies have concluded that if you grow heifers too fast before puberty, it impairs mammary development, and that causes reduced first-lactation milk yield.

However, a research analysis by Mike Van Amburgh, associate professor of dairy nutrition, and his colleagues at Cornell, debunks this popular theory. It also strongly suggests that the culprit behind reduced milk production in heifers fed elevated energy levels before puberty may very well be summed up in three words: They’re too fat.

A look at the research
In one study, the Cornell researchers found that there was no difference in mammary development between Holstein heifers fed for a higher rate of gain (2 pounds per day) and heifers fed for a lower rate of gain (1.4 pounds per day) before puberty. So, in other words, elevated nutrition did not impair mammary development. It simply caused the faster-gaining heifers to reach puberty earlier, Van Amburgh says.

Although the Cornell group did not follow heifers through their first lactation, they went back and analyzed the findings of other studies that did. Here’s a snapshot of some of that research:

A two-part study, published in the December 1995 and March 1998 issues of the Journal of Dairy Science, shows that heifers fed to gain 2.1 pounds per day experienced a 48-percent reduction in mammary development compared to heifers fed to gain 1.6 pounds per day.

However, as Van Amburgh points out, there was only a 5.4-percent difference in milk yield during the first 301 days in milk ─ a loss of 2.7 pounds of milk per day in the animals fed a higher plane of nutrition before puberty.

“Most likely it’s not the mammary gland,” Van Amburgh says.

Is extra fat to blame?
So what is responsible for reduced milk yield in first-lactation heifers that are fed for a higher rate of gain before puberty? The exact mechanism isn’t known yet, but the Cornell researchers strongly suspect that it’s because heifers pack on too much fat by the time they reach puberty.

When you feed heifers a higher-energy diet so that they gain more rapidly, they reach puberty earlier, Van Amburgh explains. However, these heifers also pack on an extra 60 to 70 pounds of fat by the time they reach puberty and even more by calving. As a result, these heifers freshen with greater difficulty, have less dry matter intake capacity and lower peak milk production.

Benchmark against your own herd
The gold standard has always been to calve heifers at 24 months of age and weighing 1,250 pounds, Van Amburgh says. The problem with that theory is it does not take into account expected mature bodyweight for cows in your herd. It’s like saying a teenager must reach 6 feet tall by the time he’s 18 years old, when his parents were only 5-feet, 2-inches tall, he adds.

“You look like your parents, and your cows should look like theirs,” Van Amburgh says.

So, if you want to improve age at first calving, set goals based on the mature bodyweight of 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.

It’s OK to grow heifers fast, Van Amburgh says, just don’t lose sight of the endpoint. Think about what you need to do in your calf- and heifer-rearing programs to achieve your age-at-first-calving goals, without getting heifers too fat.