CORVALLIS, Ore. - Cows like protein. They need a good bit to produce good milk.
But for nearly 20 years, dairy producers have been told too much protein in the diet is bad for cow reproduction.
Now Oregon State University dairy scientist Diane Carroll is stirring up the dairy industry by challenging that idea.
"If high protein is having an effect on conception rates, it's probably because the dietary protein is interacting with some other factor that is occurring," she said.
As early as 1979, scientists conducted research that showed a negative effect on cows' ability to reproduce when fed rations that contained 19 percent or more crude protein (the recommended amount is 17 to 18 percent).
"I'm sure that the results they and other scientists reported were accurate, but I'm equally sure the phenomenon doesn't happen all the time," Carroll said.
She has tested the high-protein theory three times, including in a high-producing herd in which protein exceeded 20 percent. Only when cows had an earlier health disorder did she see a decline in reproduction because of the high protein rations.
Earlier this summer at the American Dairy Science Association meetings in Ontario, Canada, Carroll and research colleague Barbara Barton from Purina Mills challenged the high-protein-hurts-reproduction idea. In front of an audience of research scientists and dairy nutritionists, they questioned research methods and urged the researchers to "look at a broader picture" if they see a slump in cow reproduction.
"We told them that on-farm assessment of protein-reproduction interaction had become too simplified," Carroll said. "People would take a milk sample, look at conception rates and say, 'You're feeding too much protein.'"
Instead, she told the scientists, they should be considering other factors that may affect reproduction - such as herd health, energy in the diet, environmental factors and accuracy of determining when a cow is in heat.
"There is definitely an interaction between cow health and protein," Carroll said. "A cow has to be vulnerable in some other way, then high protein can push her over the edge.
"There simply are a lot of reasons why a cow might be infertile."
Carroll told of a study in which she looked at a herd that was producing more than 24,000 pounds of milk per cow per year. The high producers were divided into two groups and fed different kinds of protein. One group got soybean meal; the other was fed a mix of soybean meal and fish meal. In each case, the cattle were getting a ration of 20.5 percent protein.
In that study, blood tests showed an average of 23 milligrams of urea nitrogen per 100 milliliters of blood. Scientists had been saying that anything over 20 milligrams per 100 milliliters of blood would cause cows to be less fertile.
"We didn't find that to be the case at all," Carroll said.
The study showed that the "days open" - the time elapsed between the end of lactation and calving - was 87 days for the cows fed soybean meal and 82 days for those on the fish meal-soy meal combination. Services per conception were 1.2 versus 1.4. Average milk production per day was 71.4 versus 79.8 over the first 16 weeks of lactation.
"All of these numbers are excellent. We found no negative effect on reproduction from the high protein," Carroll said.
"In that and subsequent studies, we concluded that urea nitrogen is inconsistent and unreliable in predicting cow fertility.
"We also concluded that veterinarians and nutritionists have to work together with the dairy producer to determine causes of infertility," she added.
"As for protein, the level should be set to meet the requirements of the cow, to minimize nitrogen in the waste, and to be economical."