Grazing and Greenhouse Gases:
Is the forage-fed ruminant a global warming scapegoat?

by David Boehm

Warnings of global warming had only just begun to seep into the mainstream media when cattle were first fingered as culprits. The press had a heyday with bovine flatulence - a rare chance to stir in some potty humor with the usual recipe of sex, sensation, and fear. Imagine Detroit's elation to find farting cows posted in the environmental docket with Lincolns and 5-liter Mustangs! The story was just preposterous enough to make doing anything about it seem ludicrous.

As it happens, flatulence isn't the problem. Less hilarious belches and exhalations account for 95 percent of methane emissions from ruminant animals. (So much depends on good delivery.) Nevertheless, as greenhouse gases go, methane is a major offender.

The world's 1.3 billion cows (a cow for every five humans) vent about 300,000 billion liters of methane annually. Figures from the United States Environmental Protection Agency (EPA) indicate that this source accounts for almost 20 percent of human-related methane emissions - about equal to the proportion from the natural gas and petroleum industries, and not far behind emissions from our ever-burgeoning landfills.

But does it make sense to look at ruminant methane emissions in isolation? Michael Main, a researcher at the Nova Scotia Agricultural College, doesn't think so. Main did his graduate work in this area. Some may recall his presentation to the 2001 Grazing Conference in Truro, titled "How to Be Environmentally Friendly and Make Money Too (On a Dairy Farm)."

Main has compared models of forage-intensive and confinement feeding dairy production systems, quantifying their respective financial and ecological costs and benefits. His conclusion is that well-managed forage systems can compete financially with higher-input systems, while reducing soil depletion, erosion, nitrate leaching, and emissions of greenhouse gases.

The last of these findings flies in the face of dairy science dogma, precisely because Main is looking at the whole farm and not focusing solely on ruminant methane emissions. To understand the controversy it is necessary to understand how methane production in cattle is related to that touchstone of modern agriculture, production efficiency.

The rumen, the cow's capacious forestomach, breaks down forage plants through microbial fermentation. This process, also called enteric fermentation, is similar to what yeast does to grapes in wine-making.

"In fact," Main says, "if you feed a cow enough sugar, she'll get drunk." (Don't try this at home.) A high-grain, low-fiber diet spends less time in the rumen being predigested, and produces fewer fermentation gases, including methane. Methane production, then, represents feed energy lost from the process of ruminant digestion. When methane emissions are reduced by decreasing the amount of forage in the diet, production efficiency increases. In the dairy example, this puts more feed energy into lactation.

"A cow's production will keep increasing up to about 15,000 kg of milk per year, compared to, say, 5,000-7,500 kg in a forage system," says Main. "After that the diet is so high in grain that it's just racing through her digestive system and efficiency starts to decline. . . At high grain levels you're really feeding the cow like a pig, which is really not how God designed her, or how she was adapted. " Whether cow or pig, production efficiency is a simple calculation, referred to in reverent tones as the "feed conversion ratio."

"Say you're feeding a cow 10,000 calories per day," says Main. "The feed conversion ratio is the percentage of those calories that you get back in milk or food value. The cow is going to use about 5,000 calories just to stand there and keep her body warm. If you feed her just a little above that she'll produce a small amount of milk, and that's not very efficient from a production standpoint, given that you have the gone through the expense of housing and caring for her; all you're doing is keeping a cow alive. The larger you can keep that margin above her maintenance needs, the more efficient she is from a production standpoint."

A cow with higher production efficiency will produce more milk with less "wasted" energy. This also means less methane produced through enteric fermentation. Under supply management, higher production efficiency allows a farmer to meet a certain milk quota with fewer cows, on smaller tracts of land. So it's not surprising that the Dairy Farmers of Canada's Greenhouse Gas Mitigation Program puts a lot of emphasis on recommendations to "improve milk production and feed efficiency" and to "lower the forage to grain ratio in rations."

GRAZING
Main concedes that with forage dairying you face a loss of production efficiency "as a raw 'energy in/energy out' calculation." But he believes the quest for greater production efficiency is a short-sighted approach to dairy economy and ecology - an approach that fails to profit from the biological efficiency of natural systems. High production efficiency from grain feeding comes with costs: low-level health problems such as acidosis and laminitis; higher feed costs; and environmental costs that go beyond the issue of a single greenhouse gas.

"If you have well-managed grazing, that's really cheap forage, especially if you own the landbase," he says. "Forage is the cheapest feed you're ever going to get. It's high-energy forage, too. A typical forage system is like a six-cut system, and with forages the younger they are the better the energy content and quality - less fiber, more sugars, everything that the cow needs."

Not only is the cow getting the highest-quality feed on pasture, Main argues, but the farmer enjoys higher yields per hectare from crops that are predominantly leguminous, requiring less outlay for purchased nitrogen fertilizers. These are economic factors that affect farm autonomy. The same factors affect farm ecology.

Foraging reduces the need to till the soil. Soil tillage and nitrogen fertilization not only cause soil erosion and nitrate leaching, they abet in the creation of nitrous oxide. (See "Methane" sidebar.) Leguminous forage plant species fix atmospheric nitrogen in the soil, increase biological efficiency, and reduce dependence on farm machinery. While methane emissions from enteric fermentation represent the transformation of carbon already in circulation between the earth and the atmosphere, burning fossil fuel always results in a net increase in greenhouse gases.

"It just happens," Main says, "that it takes about a kilogram (a liter) of petroleum to produce a kilogram of nitrogen fertilizer."

The corn and other high-energy feed grains that are doing their part to make milk production so "efficient" are as dependent on fossil fuels as the machines that plant and harvest and process them. Forage plants build the soil, providing a sink for carbon in the plant mass and increasing organic content of the soil itself.

"Part of the greenhouse effect," Main says, "has come from the burning of forests and the depletion of prairie organic matter. Many of our prairies are at half the levels of organic matter that they were 100 years ago."

It is this sudden degradation of the planet's ability to absorb and store carbon dioxide, coinciding with the widespread use of fossil fuels, that has created the unprecedented threat of global warming and climate change.

"In a high-forage system, you're fixing more carbon than you are leaching," Main notes, "so you're actually subtracting from the greenhouse effect."

MANURE
Pasturing animals also allows for the decomposition of manure under more-or-less aerobic conditions, flop for flop, reducing the other major agricultural source of methane emissions: manure storage. An EPA fact sheet on methane reports: "liquid manure management systems such as lagoons and holding tanks can cause significant methane production . . . (whereas) . . . manure deposited on fields and pastures, or otherwise handled in a dry form, produces insignificant amounts of methane."

Widespread use of systems for the capture and use of methane from stored manure could offset this advantage for pasturing. But due to the difficulty of storing methane - a gas that, unlike propane, cannot be easily liquefied - the cost of such systems will continue to be prohibitive for small producers, Main predicts.

Methane from belching cows makes an easy target, just as farting cows were good for a laugh. An offensive on this source of greenhouse gas happens to fit nicely with pre-existing agri-business and institutional efforts to control agricultural production and farmers themselves.

This being said, it is also true that increases in production efficiency would certainly be a boon for cattle feeding on crop residues and poor forages in arid and agriculturally poor areas of the world. R.A. Leng, an international expert consulting with the International Panel on Climate Change (IPCC), estimates that ruminants on low-quality feeds produce more than 75 percent of the methane from the world's population of ruminants.

Obviously, feeding expensive grains to India's 100 million draft oxen is a no go, but Leng believes it would make a big difference if crucial mineral supplements and molasses/urea blocks were added to their otherwise stark diet of straw and stubble. In addition to reducing emissions per head, increased production efficiency would mean the same amount of work could be done by fewer animals.

It turns out that the 15-20 percent of methane produced through enteric fermentation is just about equal to the decrease it would take to (eventually) bring methane levels back into kilter. Certainly, balancing production efficiency and cattle populations could go a long way toward achieving the reductions needed to forestall climate change.

"Best efficiency," Main's study reports, "is found in the compromise between utilizing the most energy-efficient feed source (forage) while maintaining herd production high enough to gain reasonable feed conversion efficiency."

In short, energy is always the most important part of the equation. We cannot continue to talk about "efficiencies" that ignore the profligate use of the earth's finite reserves of fossil carbon. The greenhouse effect is telling us that, even if we were to find more oil, coal, natural gas, or tar sands to exploit to keep us going on this bender, we would bury ourselves in atmosphere. The sky really is the limit, and ultimately we're going to have to come out of the laboratory to consider the lilies.

This article first appeared in the October 2004 issue of Rural Delivery.