Food

Can Chickens Eat Onions? (2022 Update) [In-Depth]

By Nick

Some people claim onions are a healthy snack. Others will tell you onions are highly toxic for chickens.

When you dig a little deeper, you will quickly discover onions are toxic for most animals. For cats, consuming as little as 0.1oz per pound bodyweight is already problematic. On the other hand, researchers investigate onions as food supplements in poultry with several health benefits.

Now what? Is onion toxic or healthy for chickens? Let's see.

What's so special about Onions?

Onions are real flavor bombs. Their signature taste is the foundation of various dishes. It's the sulfur in onions that's responsible for the punchy taste, but also for those pesky tears that pop up when you decide to cut one.

A raw onion doesn't have any odor at all. But from the moment you chop it, enzymes are released, and within 30 seconds you have an intense smell and flavor. This typical onion odor is the result of enzymes reacting with sulfur to form chemical compounds.

Why are Onions Toxic for Chickens?

Onions contain sulfoxides. When chickens digest sulfoxides, they turn into a complex mixture of sulfur-containing organic compounds. These are active chemicals that are absorbed in the intestines where they form highly reactive oxidants. Cooking the onions does not reduce the toxicity.

Sulfoxides in onions are toxic for chickens

All animals, including chickens, have red cells in their blood. These red blood cells transfer oxygen through the body. They use hemoglobin to carry the oxygen around. The sulfoxides in onion rupture the cell membrane of red blood cells, which results in a total mess. This process is called oxidative hemolysis.

Sulfoxides in onions will kill the red blood cells of chickens by creating heinz bodies

The cysteine sulfoxides of onion react with the hemoglobin to form sulfhemoglobin. This sulfhemoglobin binds itself to the cell membrane of the red blood cells and forms Heinz bodies. These Heinz bodies break down the membrane and kill the cell.

The blood cells are destroyed faster than the chicken's body can make them. As a result, oxygen can not be delivered to the chicken's organs and tissues. The onset of anemia and methemoglobinemia. These two life-threatening conditions are as bad as they sound. They can result in acute organ failure and acute respiratory failure. Fatal for the chicken.

Not Just Onions

It's not only onions that are extraordinary killing machines for chickens. All plants in the onion family contain toxic sulfoxides. Think of vegetables like garlic, shallots, leeks, and chives. They all belong to the Allium plant family.

Onion and garlic are members of the allium family

This raises even more questions. Garlic is used as a food supplement and is known to be a natural dewormer, booster for the immune system, and an active red mite repellent. But can it be toxic?

Let's stick with onions first. You might be really worried by now.

Exactly how toxic are onions?

How Toxic is Onion for Chickens?

The toxicity of onions is well-known for cats and dogs. For cats, the toxic dose is as little as 0.1oz per pound of body weight (5 g/kg). In dogs, it's 0.25-0.5oz per pound of bodyweight (15-30g/kg). That's a very small amount.

Example of onion toxicity for dogs and cats with a dog of 35lb and a cat of 8.8lb.

For animals in general, onion intoxication consistently occurs when animals ingest more than 0.5% of their bodyweight, at once. The same applies to chickens. Anything over that limit and Heinz bodies start to show up, and anemia kicks in.

This means a 4-pound White Leghorn pullet can develop anemia after eating as little as 0.32oz of onion. That's only a tiny portion of the food that the pullet usually ingests in the span of a full day. An 11-pound Brahma hen will start running into problems when she eats more than a quarter of a medium-sized onion.

Examples of onion toxicity for chickens with a leghorn of 4lbs and a welsummer of 11lbs.

Luckily chickens are no cats or dogs. Dogs and cats are incredibly susceptible to onion toxicosis since they are very sensitive to changes in their blood cells. Dogs have a less efficient oxygen production mechanism (catalase antioxidant activity), and cats are four times more susceptible to oxidative damage. That's why vets take onion intoxication in cats very seriously.

Chickens do not show such a hypersensitive reaction, so you don't immediately have to panic if you give them some leftovers from the kitchen. That said, 0.32oz will already start destructive mechanisms in the chicken's blood cells. And for those who wonder how much onion .32 oz is, this is 0.4 oz on a scale.

Example amount of onion that starts to cause trouble in a 4 pound leghorn pullet

These examples are with single doses. Problems can arise when you feed your backyard chickens regularly with leftover scraps from the kitchen. It does not matter whether the onions are raw or cooked. The toxicity stays the same.

The danger here is that it's not immediately obvious that onions aren't our bird's best friend. Most people can intuitively guess that you should not feed chickens with chocolate, but that's less evident for onions.

Toxicity for Geese

You might have heard rumors of scientific research where they accidentally killed a white Chinese goose by feeding it with onions. That's not entirely correct. The researched birds were euthanized with carbon dioxide on day 21 of the research to perform a complete autopsy of the liver and other organs.

Nevertheless, the research was initiated after two large-scale incidents in 2001 and 2002, with flocks of 1400 and 634 geese. At that moment, the adverse health effects of onions were already somewhat known, so most farmers didn't feed their birds with onions.

Farmers feeding geese with onions had an increased mortality rate

However, there were no scientific reports yet to describe the actual effects on avian species. In both cases, the geese were on a daily maintenance food supply of 5oz pellets/day/bird. But on top of that, the farmers gave them a free choice to eat as many green onions as they wanted for a week. At the end of the week, 6% and 8.4% of the geese' flocks died from anemia. Most of them were white Chinese geese.

After removing the onions from the diet, the mortality dropped back to normal. These geese had no idea that the onion was killing them from the inside.

How long before any symptoms appear after eating onions?

If a chicken has eaten onions, the poisoning process starts within the same day. Red blood cells are destroyed, and the chicken's body slowly becomes deprived of oxygen. However, this process can be prolonged, and real problems usually only appear after several days. These days can be very stressful for the owner.

Acute fatal intoxication is less common in birds compared to cats. However, chronic problems can build up if onions are a commonly available food source for several days. Similar to what happened with the farmers and the geese. Regular feeding can result in hemolytic anemia, a shortage of oxygen over time.

For chickens, it's more common to notice temporary symptoms after they eat some onion.

What are the typical symptoms of Onion Intoxication in chickens?

Some symptoms of onion intoxication in chickens are:

  • diarrhea
  • abdominal and chest pain
  • general weakness and sensitivity
  • a depressed and lethargic chicken
  • dizziness and confusion
  • increased thirst

If the anemia becomes more severe, symptoms can be:

  • pale comb and wattle
  • heavy breathing
  • poor appetite
  • irregular heartbeat and heart palpitations

When you remove onions from the diet, the regenerative response returns the body to homeostasis, and the chicken recovers.

There probably won't be any long-lasting symptoms.

Influence of Onions on Egg Taste

Sulfur is the compound that gives onions their signature taste, but it's also sulfur responsible for the smell of rotten eggs. You might think that feeding your hens onions and garlic will make them lay pre-seasoned eggs. But if anything, the sulfur will rather make the egg smell awful.

The influence of onion and garlic on the taste of eggs

Some people claim they can taste whether their chickens have some onions. Other people say they can't taste any difference. It's a common discussion among chicken owners and in the poultry industry.

Scientific Research

In fact, it's so prominent that scientists did research in 2011 to determine whether feeding onion has a significant impact on taste and egg quality.

They fed chickens with a laying meal mixed with onion. After they laid an egg, they securely distilled the volatile flavors from the egg yolks. The scientists describe the odor impressions of the purified gas as onion-like. They measured a considerable increase in sulfur per egg for the chickens that ate onions. But at the same time, they state that there is such a low abundance of sulfur in the yolk that it's very unlikely the consumer can perceive it.

In 2017, a new study was conducted in Poland that used onion extract as a food supplement. They added as little as 0.0032% of onion extract to the chicken's food. These researchers report a significant change in the aroma of the egg and the taste of boiled eggs. The effect depends on the amount of onion extract and how long your chickens get it.

Onions as Alternative to Antibiotics in Chicken Food

You might have noticed that some chicken food contains in-feed antibiotics as growth-promotor. That's great because it will keep your chickens healthy.

However, the unregulated antibiotics in chicken food introduce two substantial problems:

  • there is a risk of antibiotic residues in eggs and chicken meat
  • it's linked to the development of antibiotic resistance, a global threat to chickens as well as humans
In-feed antibiotics for chickens are being regulated

It's just a matter of time before policymakers worldwide regulate the poultry industry and limit antibiotics in chicken food. Europe already banned the sub-therapeutic use of antibiotics in animal nutrition (1831/2003/EC, 2006). That's why a lot of research focuses on finding alternatives for antibiotics.

And believe it or not, they are looking at onions and garlic to replace antibiotics.

How is that possible? Onions are toxic?

It turns out that adding tiny amounts of onion extract to their feed comes with substantial health benefits. The bioactive components in onions have some amazing effects.

Bioactive Compounds in Onions

Remember the toxic S-alk(en)yl-L-cysteine sulfoxides (ACSOs) that make onions so toxic? These are the dreaded sulfoxides that kill the red blood cells, causing our chickens to end up with life-threatening anemia.

When onions are cut, these ACSOs transform into several organosulfur compounds that are well-studied antimicrobial agents. While onions are toxic, small amounts of onion extract suddenly turn into antibacterial agents with antioxidant activity.

Apart from the ACSOs, onions also contain other bioactive compounds:

  • Saponins: biochemical compounds based on sugars that have anti-inflammatory, antibacterial, antifungal, and cholesterol-lowering activities. Saponins also suppress muscle spasms in the intestines of chickens.
  • Fructans: the specific fructans from onions and garlic have a prebiotic and antiviral effect. They boost the immune system and regulate the gut ecosystem.
  • Polyphenols: bioactive compounds in onions that have antioxidant and anti-inflammatory activities. The most important ones are probably flavonoids such as quercetin. Onions are one of the best sources of flavonoids. The health benefits of flavonoids are well known for humans. Recently scientists started to look into flavonoids to promote growth, immune, and antioxidant responses for animals.
flavonoids in onions are biochemical compounds with health benefits for chickens

Backyard Chickens

All these chemicals in onions collaborate to boost your chicken's health system. Food processors have difficulty protecting these bioactive compounds while processing and distributing the onion extracts in food supplements. They are currently looking at nanotechnology as a feed additive to provide microencapsulation of bioactive compounds.

For our backyard chickens, we can create onion extract from fresh-cut onion parts that still contain all the original bioactive chemicals. This is important because the chemical compounds start to transform and be active from the moment that the onion is chopped.

Onion extract comes with tremendous opportunities. But it remains to be seen how difficult it is to get the right dose since these investigations only add as little as 0.005% of onion extracts to the food.

Effect of Onion on the Chicken's Immune Response

Several studies have indicated that onion and garlic additives boost the chicken's immune system.

In 2010, scientists investigated the effect of onion on the immune system of one-week-old White Leghorn chicks. They added small amounts of onion to the chickens' diet (0.16oz/lb food) before vaccinating them for the Newcastle Disease Virus. They discovered that adding onions enhanced the production of antibodies without any adverse side effects. Researchers speculate that the flavonoids of onions indirectly stimulate the chicken's immune cells. In 2021, researchers found that adding vitamins A, E, or C to the diet helped boost the immune system against the Newcastle Disease virus, and since onions contain some vitamin C there might be some effect from the antioxidants.

Many studies are using other members of the onion's Allium family. Researches consistently report multiple health benefits for supplementation with garlic extracts and hooker chives in broiler chickens. It's a safe bet that these results also apply for onions since the bioactive compounds responsible for the immune response (such as flavonoids) are also present in onions.

Effects of Onion on the Chicken's Gut Microflora

The guts and ceca of chickens are full of microorganisms that make up the chicken's microbiome. There are more bacterial cells in a chicken than normal body cells. They play a fundamental role in the digestion, immune system, and vital health systems of a chicken. The most common bacteria in chickens are the same we carry around in our human intestins. These include Lactobacillus, Streptococcus, Escherichia, Salmonella, and Campylobacter.

the chicken's microbiome

Recent studies have found a lot of beneficial gut regulating effects when adding onion to a broiler chicken's diet. Onion supplements caused a significant reduction of the E. Coli bacteria, a common cause of infections. At the same time, it increased the amount of Lactobacillus, a common probiotic and one of the reasons many people are adding yogurt to their diets.

These changes increase the absorption of nutrients, the immune system, growth performance, and the chicken's overall health.

Onion as Growth Stimulator for Chickens

Some studies report that adding 1% of onion extract in the drinking water of chickens increased their growth performance. However, several studies could not observe this effect on growth or bodyweight gains when adding onion to the chicken's diet.

Some researchers believe that the growth-promoting supplements will only affect chickens living in stressful confinement. Our well-nourished, healthy backyard chickens that live in ideal conditions will probably not experience a sudden growth spurt after eating onions.

Influence of Onions on Egg Quality and Egg Production

Chicken eggs are known to be very responsive to any dietary changes. Several studies report a significant improvement in egg quality for vegetables in the allium family.

In 2017, researchers did an extensive study on 216 laying hens to investigate the effect of onion supplementation in their diet. The study took 17 weeks and tracked everything from yolk color to shell quality. There was a clear improvement in fat composition, and they ended up with heavier eggs. These eggs had more egg yolk and a higher quality of albumen, the white of the egg.

Adding onion juice supplementation to the food of chickens increased the egg quality and size of the eggs

In 2019, a Czech research team did another large study on 240 Babcock laying hens. They found that onion juice supplementation of 2% in drinking water increased egg production. Adding as little as 0.5% of onion juice to drinking water already showed a remarkable increase in egg weight. Adding as low as 0.0032% of onion powder to the diet increased the egg weight by ~2oz (56g). However, it also delayed egg production. Interestingly, this effect was only observed with chickens. They could not reproduce it for Japanese quail hens.

Nutritional Benefits of Onions

We've covered the bioactive properties of onions in-depth, but would onions be a good food source in general? Do they contain any vital nutrients, or are they just empty vegetable blobs with toxins and little nutritional value like rhubarb leaves?

An onion is basically 90% water with a little fiber (1.7%) and almost no carbohydrates (0.35%), proteins (1.1%), or fat (0.25%). It's very low in calories and does not contain any macronutrients. Onions won't help you too much to survive.

However, onions are a good source of minerals and vitamins. They contain lots of vitamin C, B11, B6 and are a great source of Kalium, an essential mineral that plays a role in cell regulation, the nerve system, and muscle function. They bring an interesting palette of micronutrients to the table without adding too many calories. Their composition is pretty consistent in all varieties of onions in the world.

So, besides bioactive chemicals like polyphenols, onions also contain other vital micronutrients for chickens. This is a win because the juice extracted from onions will still contain most vitamins, minerals, and phytonutrients.

Do Chickens Eat Onions?

Some chickens love onions and will happily devour every part of them. If you present them with kitchen leftovers, they will eat them. If you have wild onions, they will eat the greens, the white flowers, and the bulbs.

However, it's up to the chicken's preferences. Some chickens won't touch any onion at all. They will glance at them and decide it's nothing for them. They will eat everything else but leave the onions out.

But if they do, the ultimate question remains.

Can Chickens Eat Onions?

No, chickens can't eat onions. They can tolerate tiny scraps of leftovers from the kitchen. The paradox is that they can also benefit from onion extracts added to their food or water. The onions' bioactive compounds have substantial health benefits and improve egg quality and production. However, chronic feeding with onions is a terrible idea since toxic sulfoxides will destroy red blood cells and lead to anemia, a severe condition that can be fatal for your chickens.

If you want to learn more about chicken feed, please consult our 'Chicken Food Page' to go and see every specific food article we address. Including all articles on what chickens can and can not eat. Or go to our listicle food summary on 'The Classroom'.

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Nick

Nick is a chicken fan, writer, and an all-time chicken fanboy holding a master in engineering. He has a passion for chemistry and he loves running with his border collie that's protecting his flock.

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