"It can be a hot topic in Oklahoma for any number of reasons, not the least of which is that the state ranks third nationally in the number of beef cows," he said. "The truth is there are tradeoffs in different aspects of sustainability when comparing the two finishing production systems."
Grain-finished beef has a lower carbon footprint than grass-finished beef because of cattle's more efficient utilization of feed in the finishing phase, fewer days on feed and greater amount of beef produced per animal.
However, grass-finished beef contributes to sustainable beef production by utilizing forage resources to produce food from plants that are largely inedible by humans.
The carbon footprint for beef is all the greenhouse gas emissions produced during the production of beef divided by the total amount of beef produced throughout the system's three phases: cow-calf, stocker/backgrounding and finishing.
"Cattle entering the final 'finishing' stage are typically 12 to 16 months of age, and remain in this phase until they achieve a level of body condition that will provide a positive eating experience for consumers," Rusk said.
The main difference in carbon footprints between grass- and grain-finished beef occurs as a result of the time spent in the finishing phase, the type of feed consumed and the body weight of the animal at the end of the finishing phase.
Typically, cattle entering the feedlot for finishing eat a diet containing corn along with byproducts such as distillers grain left over after ethanol production or corn gluten feed left over after corn fructose production, as well as vitamins and minerals and small quantities of roughage such as hay.
"Grain-finished cattle remain in the feedlot for approximately four to six months, and reach market weight faster than their grass-finished counterparts because their diets are higher in energy, which results in rapid and efficient weight gain," said Sara Place, assistant professor of sustainable beef cattle systems for OSU's Division of Agricultural Sciences and Natural Resources.
In contrast, grass-finished cattle gain weight at a slower rate and typically go to harvest at 20 to 26 months of age, and at a lower final weight than their grain-finished counterparts.
"Grass-finished cattle may finish either faster or slower than the typical age range depending upon the forage and grass resources available," Place said. "For example, the growing season is shorter in northern states, which may shorten the finishing period and lead to lighter animal weights at harvest."
The difference in harvest weights translates into different numbers of U.S. citizens who can be fed per animal.
Place said average grass-finished cattle provide a live animal weight at harvest of 1,100 pounds, a dressing percentage of 58 percent and a carcass weight of 638 pounds per animal resulting in eight U.S. citizens being fed per animal, according to USDA per capita beef consumption data.
"The dressing percent of an animal represents the meat and skeletal portion of an animal compared to its live weight," Place said. "Utilizing grass forage as the primary source of feed also contributes to an increased carbon footprint because high forage diets produce more methane emissions from the animal's digestive tract than high-energy grain diets."
Methane is a greenhouse gas 28 times more potent at trapping heat in the earth's atmosphere as compared to carbon dioxide.
For grain-finished cattle with a live animal weight at harvest of 1,300 pounds, a dressing percentage of 64 percent and a carcass weight of 832 pounds per animal would result in 10.4 U.S. citizens being fed per animal, according to USDA per capita beef consumption data.
"The combination of consuming a higher-energy, lower-forage diet; less time spent on feed during the finishing phase; and heavier carcass weights translates into an 18.5 percent to 67.5 percent lower carbon footprint for grain-finished beef compared to grass-finished beef," Place said.
So why is there a debate, some might ask?
"Even though grass-finished beef has a higher carbon footprint, it also has some sustainability advantages," Place said.
Grass-finished cattle utilize foodstuffs that are inedible to humans as their primary source of energy and nutrients throughout their lifetimes, but it doesn't end there.
"Beef cattle can utilize forage grown on land that is not suitable for crop production, thus providing us humans with a nutritious food source on land resources that otherwise would not be providing something we can eat," Place said.
Additionally, grasslands and pastures can sequester carbon dioxide from the atmosphere, which can help mitigate climate change, as reported by the Intergovernmental Panel on Climate Change.
Research conducted by Pelletier and Rasmussen in 2010 also shows there is an advantage for grass-finished beef production compared to grain-finished beef production when expressing feed conversion as human-edible energy returned per unit of human-edible energy consumed by the cattle.
"Accounting for carbon sequestration could lower the carbon footprint of grass-finished beef by 42 percent," Place said. "In contrast, approximately 18 percent of feed intake per unit of carcass weight will occur in the feedlot for grain-finished cattle."
The bottom line is this: Beef cattle producers using either system can sustainably meet consumer demand for beef.
"It is human nature to discuss and even debate topics about which we are passionate, and to have some sort of 'winner,'" Rusk said. "OSU's land-grant university mission is to determine and disseminate research-based factual information so people are better informed and can make knowledgeable decisions about issues and concerns of importance to them, their families and their communities, even in instances where there is no clear winner."
Cattle and calves account for approximately half of agricultural cash receipts in Oklahoma, according to USDA National Agricultural Statistics Service data.