Research shows that injecting probiotics into chicken eggs before hatching does not improve growth performance, and adding probiotics to feed actually reduced growth during the first four weeks of life in a 2026 study of 504 broiler chicks. According to Gram Research analysis, chicks receiving dietary probiotic supplements at 5 to 10 milliliters per kilogram of feed weighed less and ate less than control chicks, suggesting that the probiotic products tested may not be effective growth-promotion tools for meat chickens
Researchers tested whether giving chickens probiotics (helpful bacteria) before they hatched and in their feed would help them grow bigger and healthier. They injected probiotics into 540 eggs before hatching and fed some chicks probiotic-supplemented feed. According to Gram Research analysis, the early probiotic injection didn’t improve how fast the chickens grew or how much they ate. Surprisingly, adding probiotics to the feed actually made the chickens grow slower during the first few weeks. The study did find that the early injection changed the types of bacteria living in the chickens’ intestines and boosted some protective substances in their livers, but these benefits didn’t translate into healthier or bigger birds.
Key Statistics
A 2026 study of 504 broiler chicks found that in ovo probiotic injection did not improve body weight, feed intake, or feed conversion ratio throughout the 42-day experimental period compared to control chicks
Dietary probiotic supplementation at 5 to 10 milliliters per kilogram of feed reduced body weight starting in the second week of life and decreased feed intake during days 8 through 28 in the 2026 broiler chicken study
In ovo probiotic injection increased total antioxidant capacity in liver tissue but reduced vitamin E, retinol, and carotenoid concentrations by day 42 in the 504-chick study
The 2026 research found that in ovo probiotic injection increased Bacillus subtilis counts while decreasing Lactobacillus species and Saccharomyces cerevisiae in broiler intestinal contents, demonstrating microbial changes without growth benefits
The Quick Take
- What they studied: Whether giving chickens probiotics before they hatch and in their feed helps them grow faster and stay healthier
- Who participated: 540 fertile chicken eggs that hatched into 504 chicks, divided into six different treatment groups to compare different probiotic strategies
- Key finding: Injecting probiotics into eggs before hatching did not improve chicken growth, weight gain, or feed efficiency. Dietary probiotic supplements actually slowed growth during weeks 2 through 4 of life
- What it means for you: If you raise chickens, injecting probiotics into eggs before hatching may not be worth the cost or effort for improving meat production. The dietary supplements tested here showed no benefit and may even slow growth. However, this was one study in one chicken breed, so farmers should consider consulting with poultry experts before changing their practices
The Research Details
Researchers started with 540 fertile chicken eggs from a common meat-producing breed called Ross 308. On day 18 of incubation (when the chicks were still developing inside the eggs), they either injected a tiny amount of probiotic solution (50 microliters, about the size of a small drop) directly into some eggs or left others without injection. After the chicks hatched, they divided the 504 surviving chicks into six groups. Some groups ate regular feed, while others ate feed mixed with probiotics at two different amounts (5 or 10 milliliters per kilogram of feed). Each treatment group had four separate pens of chickens to make sure the results were reliable.
The researchers tracked how much the chickens weighed, how much they ate, and how efficiently they converted feed into body weight over the entire 42-day study period. They also tested the chickens’ liver tissue to measure antioxidants (protective substances that fight damage in the body) and examined the bacteria living in their intestines to see which types were present and in what quantities.
This experimental design, called a factorial arrangement, allowed researchers to test two different probiotic strategies at the same time and see if they worked better together or separately. By using multiple pens per treatment group, they could account for normal variation between individual chickens and be more confident in their results.
This research approach matters because it tests probiotics at two critical times in a chicken’s life: before birth (when the immune system is developing) and after hatching (when the digestive system is establishing its bacterial community). By measuring multiple outcomes (growth, antioxidants, and intestinal bacteria), the researchers could see whether probiotics work through different mechanisms. The large sample size and controlled conditions make the results more reliable than smaller or less controlled studies. Understanding whether early probiotic intervention actually improves chicken health and production is important for the poultry industry, which is constantly looking for ways to improve efficiency and reduce the need for antibiotics.
This study has several strengths that make its findings reliable. The researchers used a large sample size (504 chicks), multiple treatment groups, and replicate pens to reduce the chance of random variation affecting results. They measured multiple outcomes rather than just one, which gives a more complete picture. The study was published in a peer-reviewed scientific journal, meaning other experts reviewed it before publication. However, the study tested only one chicken breed and one type of probiotic product, so results may not apply to all chickens or all probiotics. The study also didn’t measure whether the probiotic changes affected disease resistance or other health markers beyond what was tested. Additionally, the negative effects on growth from dietary probiotics were somewhat surprising and deserve follow-up research to understand why they occurred
What the Results Show
The main finding was that injecting probiotics into eggs before hatching did not improve how chickens grew. Chicks that received the in ovo (before hatching) probiotic injection weighed the same as control chicks throughout the 42-day study period. They also ate the same amount of feed and converted feed to body weight equally well. This was true whether or not they also received probiotics in their feed after hatching.
Surprisingly, adding probiotics to the feed after hatching actually had negative effects on growth. Starting in the second week of life, chicks fed probiotic-supplemented feed weighed less than chicks eating regular feed. During the first three weeks of life (days 8 through 28), chicks on probiotic-supplemented feed also ate less food. The higher the probiotic dose in the feed (10 milliliters per kilogram), the more pronounced these negative effects appeared to be.
The researchers also measured antioxidants in the liver, which are protective substances that help prevent cellular damage. The in ovo probiotic injection increased total antioxidant capacity in the liver. However, this protective effect came with a trade-off: the injection reduced levels of specific antioxidants including vitamin E, retinol (a form of vitamin A), and carotenoids (plant pigments with protective properties). By day 42, chicks that received the in ovo injection had lower concentrations of these important nutrients in their livers.
The study found that probiotics did change which bacteria lived in the chickens’ intestines, even though this didn’t improve growth. The in ovo injection increased Bacillus subtilis (a type of beneficial bacteria) but decreased Lactobacillus species and Saccharomyces cerevisiae (a type of yeast). Dietary probiotic supplementation at the highest level (10 milliliters per kilogram) also increased Bacillus subtilis while decreasing Lactobacillus species. These changes show that the probiotics did colonize the intestines and alter the microbial community, but the altered community did not translate into better growth or feed efficiency. This suggests that simply changing the types of bacteria present may not be enough to improve chicken performance if the new bacterial community doesn’t provide functional benefits
Previous research on probiotics in poultry has shown mixed results. Some studies reported improvements in growth and immune function, while others found minimal effects. This study’s finding that dietary probiotics actually reduced growth is noteworthy and somewhat contradicts the optimistic results from some earlier research. The finding that in ovo injection didn’t improve growth differs from a few previous studies that suggested early probiotic intervention might be beneficial. However, the study’s results align with growing recognition in the scientific community that probiotics don’t automatically improve performance just because they establish themselves in the gut. The specific probiotic strains used, the timing of administration, and the chicken breed all appear to matter significantly. This research adds important evidence that not all probiotic interventions work as expected and that some may even be counterproductive
This study has several important limitations that readers should understand. First, it tested only one specific probiotic product and one chicken breed (Ross 308), so results may not apply to other probiotics or other chicken breeds. Second, the study lasted only 42 days, which is the typical lifespan for meat chickens but doesn’t tell us about long-term effects. Third, the researchers didn’t measure disease resistance or immune function directly, so we don’t know if the probiotic changes affected the chickens’ ability to fight infections. Fourth, the study didn’t explain why dietary probiotics reduced growth, which is an important question for future research. Finally, the study measured only culturable bacteria (bacteria that can be grown in the lab), which represents only a fraction of the total bacterial community in the intestines. Modern DNA-based methods can identify many more bacteria, so the full picture of microbial changes may be more complex than what this study revealed
The Bottom Line
Based on this research, poultry farmers should be cautious about using probiotics as a growth-promotion strategy in broiler chickens. The evidence does not support injecting probiotics into eggs before hatching as a way to improve growth performance. Dietary probiotic supplementation at the doses tested (5 to 10 milliliters per kilogram of feed) actually reduced growth and feed intake, so these specific products and doses should not be used for growth promotion. However, probiotics might still have value for other purposes not measured in this study, such as improving disease resistance or gut health markers. Farmers considering probiotics should consult with poultry nutritionists and veterinarians, request evidence specific to their chicken breed and production goals, and conduct small-scale trials before implementing changes farm-wide. Confidence level: Moderate. This is one study in one breed, so results should be confirmed in other settings before making major changes
Poultry farmers and producers who raise broiler chickens for meat should pay attention to these findings, as they directly affect production efficiency and profitability. Feed manufacturers and nutritionists who formulate chicken diets should consider these results when recommending probiotic products. Veterinarians who advise poultry operations should be aware that the probiotics tested here did not improve growth and may have reduced it. Consumers interested in how chickens are raised may care about this research if they prefer products raised without unnecessary supplements. Researchers studying probiotics in animals should note these findings as evidence that probiotic effects are not universal and that careful testing is needed for each product and application. People raising backyard chickens should understand that commercial probiotic products may not deliver the growth benefits sometimes claimed in marketing materials
Frequently Asked Questions
Do probiotics help chickens grow faster and bigger?
In this 2026 study of 504 broiler chicks, probiotics did not improve growth. In fact, dietary probiotic supplements reduced body weight and feed intake during the first four weeks of life, suggesting these products may not be effective for promoting chicken growth
Is it worth injecting probiotics into chicken eggs before they hatch?
According to this research, in ovo probiotic injection did not improve body weight, feed intake, or feed conversion ratio in broiler chicks. The injection did change intestinal bacteria and liver antioxidants, but these changes did not translate into better growth performance, making the practice questionable from a production standpoint
What probiotics should I add to chicken feed for better health?
This 2026 study found that the probiotic supplements tested actually reduced growth performance in broiler chickens. Before adding any probiotics to feed, consult with a poultry veterinarian or nutritionist about products with evidence specific to your chicken breed and production goals, as results vary significantly
Can probiotics replace antibiotics in chicken farming?
This study did not measure disease resistance or antibiotic alternatives. While probiotics might have other benefits not tested here, this research shows they did not improve growth performance. Decisions about replacing antibiotics should involve veterinary guidance and evidence specific to disease prevention, not just growth promotion
How long does it take to see benefits from chicken probiotics?
In this 42-day study, dietary probiotics showed negative effects on growth starting in week two and continuing through week four. If probiotics were going to help, benefits would likely appear within the first few weeks, but this research found no positive growth effects at any time point tested
Want to Apply This Research?
- If tracking poultry production, measure and record weekly body weight per bird and total feed consumption per pen. Compare these metrics between flocks using probiotics and flocks using standard feed to see if the pattern from this research holds true on your farm. Track these measurements for the full 42-day grow-out period
- If you currently use probiotics in broiler feed, consider conducting a small trial where you compare a control group (standard feed) against your current probiotic protocol. Measure weight gain and feed efficiency in both groups over 4 to 6 weeks. Use the app to log weekly weights and feed amounts, then calculate feed conversion ratio to see if probiotics are actually delivering value on your operation
- Over a 12-week period, track growth performance metrics for each flock you raise. Log body weight at weeks 2, 3, 4, and 6 of age, total feed consumed, and calculated feed conversion ratio. Compare results between flocks receiving probiotics and control flocks. If you see the same pattern as this research (slower growth with probiotics), you have evidence to discontinue the practice. If you see different results, document them as evidence that your specific situation may differ from the research
This research summary is for informational purposes only and should not replace professional veterinary or nutritional advice. The study tested specific probiotic products in one chicken breed over 42 days, so results may not apply to all probiotics, all chicken breeds, or all production systems. Farmers considering changes to probiotic use should consult with a licensed veterinarian or poultry nutritionist who understands their specific operation. Individual results may vary based on management practices, environmental conditions, and other factors not controlled in this research. This summary does not constitute medical or veterinary advice for any specific situation
This research translation is published by Gram Research, the science division of Gram, an AI-powered nutrition tracking app.