"That's one of nature's wonders," Hristov said.
Yet the by-products of the animal's digestive system also are among nature's problems, both in the air and in impaired water bodies such as the Chesapeake Bay. Hristov, Lee, and their colleagues are experimenting with sophisticated diets to reduce harmful pollutants that emerge from, ahem, both ends of the cow.
The stakes are high. The U.S. Environmental Protection Agency has proposed mandatory reductions in pollution from Pennsylvania and the five other states in the bay's watershed, each of which is in the midst of determining where the cuts will be made.
Agriculture is a prime target. It accounts for a large share of pollution, in the forms of nitrogen, phosphorus, and sediment. Livestock manure is loaded with nitrogen, which is useful as fertilizer but is an environmental threat when it washes into waterways. Along with phosphorus, nitrogen fosters the growth of excess algae, eventually robbing the water of oxygen.
Some farmers have reduced this runoff pollution by planting vegetative buffers and erecting fences to keep cattle away from streams. Yet of the 250 million pounds of nitrogen that pollute the Chesapeake watershed each year, farms still account for more than 100 million pounds, according to EPA estimates - more than 40 percent of the total.
Enter the scientists.
Cows consume nitrogen in the form of protein in their feed. Some of it is used to produce milk. More than half is excreted as waste.
The Penn State scientists asked: Could they reduce the amount of protein in the diet - and thus the amount of nitrogen in the cow's manure - without affecting milk production?
So far, the answer seems to be yes.
Hristov and his colleagues are conducting several experiments, funded by a $226,000 grant from the U.S. Department of Agriculture.
They have fed cows a diet that contains 14 percent protein, down from the typical 17 percent or more. They also supplemented the diet with small amounts of two amino acids, methionine and lysine, that are essential for milk production.
"We know more about feeding cows than people," Hristov joked.
Milk production remained steady in a group of 36 cows on the special diet, although the concentration of protein in the milk decreased slightly. A larger, 120-cow study is scheduled for the fall.
Farmers have experimented with lower-protein diets before, primarily to make their livestock more efficient, said Glen A. Broderick, a dairy scientist at the USDA's Dairy Forge Research Center in Madison, Wis.
"You can maybe formulate your diet for less money," Broderick said.
Increasingly, diets are tweaked for environmental reasons as agriculture comes under more regulatory scrutiny, but the 14 percent protein formula at Penn State is unusually low, Broderick said.
Whether science alone can get the job done remains to be seen. Pennsylvania has 550,000 dairy cows, according to the National Agricultural Statistics Service, and environmentalists warn that some farmers do not have enough land to accommodate the animals' manure regardless of its nitrogen content.
"There are definitely large pockets in the state where there are way too many nutrients for the land base," said Kim Snell-Zarcone, an agricultural and water policy analyst for the environmental group PennFuture.
Cows pollute the air as well.
Nitrogen in urine can break down to form ammonia, a gaseous pollutant that hampers air quality. And methane, a greenhouse gas that is blamed for global warming, comes out the cow's front end - through belching.
Once again, the Penn State team is trying to address the issue through diet. One feed additive that has shown early promise might seem more at home in a good marinara sauce: oregano.
In a small group of cows that ate food seasoned with the herb, methane emissions declined by 40 percent - though Hristov cautioned that he must repeat the experiment to make sure it wasn't a fluke.
First, the scientists took samples of material from a cow's rumen - one of its four stomachs. They extracted the material through the hole that had been surgically cut in the animal's side; it is easily closed up again with a plastic plug, causing no apparent distress to the animal.
The scientists then took the rumen material and added 200 different plant and herbal compounds, to see which ones generated the least methane.
Oregano was among the best, so they tried feeding it to actual cows. Hristov said he had already applied for a patent on this odd dietary supplement, in case the early findings from the dairy barn hold up.
If nothing else, it lends the place a welcome hint of fragrance.
Contact staff writer Tom Avril at 215-854-2430 or email@example.com.