Methane emissions from ruminants do play a factor in a warming planet. But there are feeding options being researched that could help reduce that impact. Unless you’ve been living on Neptune for the past 10 years, you know about the issue of climate change (on Earth).
Greenhouse gases – carbon dioxide, methane, nitrous oxide – are all increasing in the atmosphere, trapping heat energy, causing temperatures to rise. But global warming has thrust livestock agriculture directly into the crosshairs of some activist groups and sections of the popular press.
Fingers are pointing at us. Why? Because a high percentage of agricultural methane comes from ruminants: sheep, cattle and goats. These animals eat, they digest feed in the rumen, and the rumen microbes release methane into the atmosphere. Methane is a natural product of rumen fermentation. The question is: How can we reduce the amount released into the atmosphere? Let’s examine one intriguing possibility.
First, some background. Methane is one of the simplest organic molecules. One carbon atom bonded to four hydrogen atoms. Methane is a colorless, odorless gas produced from geological, industrial and biological sources. In the biological realm, microbes create methane when organic matter rots without oxygen (anaerobic fermentation). This occurs in landfills, swamps and permafrost. Melting permafrost releases methane. (This itself is an important issue in climate change.) Methane is also produced in rice fields, the digestive tracts of termites and the rumens of ruminants.
The critical issue for us is: Methane is a major greenhouse gas. Although individual methane molecules remain in the atmosphere for only 10 to 12 years, their cumulative heat-trapping effects over time are 30 times more potent than carbon dioxide. Major reports on climate change list global methane numbers and percentages all over the map, but in the U.S. today, there’s no question its 95 million cattle, 5 million sheep and 2 million goats produce a lot of methane. That has made ruminant agriculture a vulnerable target.
Feeding bugs
All ruminants have a rumen, which is really a large fermentation vat teeming with bacteria, protozoa and other microbes in an incredibly complex and dynamic ecosystem. Basically, it’s a flow-through septic tank filled with rumen bugs. Once feed enters the rumen, rumen bugs ferment it into microbial products that either flow further down the gastrointestinal tract or are absorbed directly across the rumen wall into the blood. The digestible carbohydrates in the feed – starches, sugars, fiber, etc. – are fermented into small simple molecules called volatile fatty acids (VFAs) which are quickly absorbed. The main VFAs are butyrate (four carbons), propionate (three carbons) and acetate (two carbons).
A critical characteristic of the rumen is: Its fermentation is anaerobic – no oxygen. Instead, the rumen contains lots of hydrogen ions. The biochemistry is complex, but the underlying metabolic pattern in the rumen is that feed energy moves down the carbon chain, ending in acetate.
But there’s more. Acetate has two carbons, and specialized rumen microbes can split those carbons, add hydrogen atoms and create a one-carbon molecule: methane. This reaction also produces carbon dioxide which the microbes can then combine with additional hydrogen atoms to form more methane (and water). Since methane is essentially a waste product, it then escapes the rumen as a gas.
The popular press merrily labels this process as “burping” or “belching,” but scientists prefer the more dignified, scientific-sounding term “eructation.” The microbes that produce methane are called “methanogens.” These single-celled micro-organisms are neither bacteria nor protozoa; they belong to a separate group called “archaea.” This is quite interesting because, as a group, methanogens can be vulnerable to suppression compounds. This is precisely the focus of much research, as scientists are actively seeking ways to suppress rumen methanogens. The good news is: They may have found something that does this.
Which is … (drumroll) … garlic. Huh? Yes, the same universal spice plant we find in every supermarket; the same garlic plant that has been used in medicinal potions since the Roman Empire. (Pliny the Elder was an enthusiastic proponent.) But unlike the medical community, our interests are methane and the effects of garlic on the rumen methanogens.
A taste of garlic
We all know garlic has a distinct and rather pungent flavor due to the compound allicin, which is released when the garlic clove is crushed or chewed. Garlic contains quite a few bioactive compounds, but allicin is probably the main reason for the historical medical interest. Allicin has some antibiotic properties, and it may also have effects on the cardiovascular system.
Let’s detour for a moment into terminology. When you read about experiments, you’ll routinely come across two terms: in vitro and in vivo (which are often written in italics). Essentially, in vitro means the study was done on a laboratory benchtop using glassware like test tubes, beakers, etc. In vivo means the study was done in a living animal. In vivo trials, of course, are the gold standard. They are also much more expensive (and risky) than in vitro trials, but that’s how science works.
For example, scientists will conduct in vitro experiments with lots of drugs to monitor how those compounds affect bacteria or whatever. But for any promising drug, they always follow up with in vivo trials because the body is far more complex than a glass beaker, and they must test if the drug actually works in a real-life situation. This is also true with rumen research. Scientists have long conducted in vitro experiments using artificial rumens made from flasks, tubes and other sophisticated laboratory equipment.
But if any in vitro results are promising, those compounds must then be tested in living sheep or cattle. (Sheep are usually less expensive.) Over the past 10 years, researchers have identified several compounds that suppress methanogens in vitro, but most of these compounds haven’t panned out during in vivo trials. A living rumen is far more complex than a flask. Its ecology is dynamic and constantly changing, and rumen microbial populations can adapt over time. So here I’ll describe some new research on garlic because it’s the one substance that has shown promise in vivo – in live animals.
A closer look
In a research station in China, scientists conducted two small trials with yearling ewes fed hay-grain diets for between 29 to 42 days. Each day, they supplemented the ewes with 2 grams of allicin. At various times during the trials, they made detailed physiological measurements – and found a 6% to 10% reduction in methane release, increased levels of rumen VFAs and increased fiber digestibility. They reported this in 2016.
In a much larger study on a private dairy farm in Scotland, researchers fed a commercial product containing garlic powder to 396 lactating cows. The dose rate was 15 grams of the product per cow per day. They measured methane release before, during and after the 12-week period of feeding the supplement. They reported in 2019 that the garlic powder reduced methane by approximately 30% and also increased milk yield.
These in vivo studies are just initial trials, of course, but they are definitely promising. The garlic supplement clearly reduced the amount of methane released into the atmosphere. The concomitant increase in rumen VFAs (sheep trial) and milk production (cow trial) fit together like the pieces of a jigsaw puzzle. Reducing the loss of methane, which is a waste product, allowed the rumen microbes to redirect more feed energy into metabolites (VFAs) that the animals could use for increased production. And importantly, these observations occurred over many weeks, which suggests the garlic effects were not diminished by adaptations or changes in the rumen microbial environment.
Whew! That’s quite a slug of rumen physiology and biochemistry. But let’s explore this option a little further. If future experiments confirm these results, we might have a tool that effectively reduces rumen methane production while potentially increasing animal production.
In a practical sense, garlic may be an ideal feed additive. It’s a natural plant that can be grown on farms, even organic farms. It’s universally used in human foods. It’s clearly not toxic. The FDA would probably classify it as GRAS (Generally Recognized As Safe) and would not require years of testing. Private companies would not need to invest vast amounts of money in expensive industrial equipment. And only a tiny supplement in a ration could have a huge effect on reducing rumen methane emissions.
So what’s the catch? Well, we still need to conduct lots of on-farm trials to corroborate those initial results. We will need to identify the best ways of adding garlic to diets. We will need companies willing to make investments in refining, packaging and distributing feed-grade garlic. And we might need to devise a solution to a vexing practical situation: How would we deal with a barn filled with 200 steers that have garlic breath?
A spicy problem indeed.
