Over the past couple of years, when chatting with people who raise chickens, eight out of ten say the same thing: chickens are harder to raise than before.
It’s not your imagination. Breeding priorities are changing, farming density is changing, and pathogens are changing too. Commercial breeding programs are investing unprecedented resources in disease resistance. Molecular markers, whole-genome selection, and even gene editing have all entered the practical stage, and gene-edited chickens resistant to Salmonella have already been successfully bred in China. But it takes several years for progress at the breeding end to reach commercial chicken farms. The more immediate issue is that seed-stock diseases continue to amplify along vertical transmission chains, immunosuppressive diseases continue to mutate in the field, and, after the ban on antibiotics, the gut barrier has lost one more layer of “extra protection.”
The broader environment has changed, so our approach to immunity must change too. The days when a few bottles of multivitamins and a few vaccine shots could “cover all the bases” are over.
The first line of defense: clean up the breeding stock and don’t let chickens “arrive carrying disease”
The most frustrating thing about hereditary and vertically transmitted diseases is that by the time chicks arrive at the farm, they may already be starting at a disadvantage.
Pullorum disease, avian leukosis, infectious anemia, and Salmonella share a common pattern: breeding chickens carry the pathogens, hatching eggs carry the disease, and chicks are already at a disadvantage in their first few days, before their immune systems have fully developed. Avian leukosis is especially typical: it not only causes disease itself, but also “teams up” with other immunosuppressive diseases, creating a disease burden in the flock that cannot be shaken off.
The first thing to do is treat “sourcing breeding stock” as the starting point of immunity management, not the end of the procurement process. Give priority to breeding stock with a record of disease eradication, and request disease-eradication test reports for the breeder flock before chicks arrive. Pay particular attention to the positive rates for pullorum disease, avian leukosis, and infectious anemia. Experience at the Yugukou Dongwa Base has shown that pullorum disease eradication is not a question of whether it can be done, but whether a testing program can be established: conduct the first whole-flock test when the laying rate reaches 10%–30%; once the positive rate meets the target, switch to monthly sampling; test 100% of roosters each month; and, combined with single-family paper-bag management during incubation and “one chicken, one tube” insemination, the vertical transmission chain can be substantially broken. Commercial chicken farms cannot achieve the same level of disease eradication as grandparent farms, but they can at least do one thing: keep a “breeding-stock source record” for each batch of chicks arriving at the farm and track at least three consecutive batches. This will give them a clearer picture of the consistency of breeding-stock quality.
The second line of defense: with immunosuppressive diseases, the goal is not to prevent “disease,” but to prevent the “foundation from being dismantled”.
Bursal disease, infectious anemia, reovirus, and infectious bronchitis—these names are familiar to poultry farmers. But many focus on whether disease breaks out and whether chickens die, overlooking more hidden damage: once the bursa atrophies, the immune system’s “military academy” is wiped out, and every vaccine administered afterward is only half as effective despite twice the effort.
The bursal disease viruses currently seen in the field are no longer dominated by a single strain. Variant strains account for nearly seven tenths, with very virulent strains and classic strains coexisting. The trouble with variant strains is precisely that they do not kill chickens and cause no typical signs, yet the bursa still atrophies and the antibody response to Newcastle disease still declines. The body weight of broilers at 42 days of age can differ by more than 400 grams. This means that if you still judge whether bursal disease has been controlled by whether any chickens have died, the judgment itself is no longer valid.
In practice, two adjustments are needed. First, shift the immunization program from a “fixed calendar” to a dual basis of “antibody levels + strain matching.” In endemic areas, consider introducing a VP2 subunit vaccine or an immune-complex vaccine at around 1 or 7 days of age; do not use the same program from the beginning to the end of the year. Second, treat immunosuppressive diseases as “targets for coordinated prevention and control” rather than isolated diseases: if bursal disease is not well controlled, antibody uniformity for Newcastle disease and infectious bronchitis will inevitably suffer; in flocks positive for infectious anemia, the response to other vaccines will also be reduced. When testing antibodies, do not look only at the average; focus on the dispersion. A high average antibody level with uneven results often indicates that the immune systems of a group of chickens are already “working while ill.”
The third line of defense: the environment is the cheapest immune investment, and the easiest to exhaust
Many chicken farmers are willing to spend money on vaccines and medicines, yet try to “save costs” on ammonia. But the respiratory mucosa is a chicken’s first physical immune barrier. When the ammonia concentration exceeds 10 ppm, the clearance function of the mucosal cilia begins to be impaired, and sIgA is directly damaged. Long-term exposure to even low concentrations causes irreversible suppression of mucosal immunity. In winter, reduced ventilation to retain heat lets ammonia build up, followed by respiratory disease, which is then suppressed with antibiotics—a familiar approach that is no longer viable under the antibiotic ban.
The targets need not be complicated; just keep to a few firm limits: ammonia below 10 ppm, humidity at 50%—65%, dust settled through in-barn spraying, and manure ammonia production reduced at the source through litter management and gut health. The conflict between ventilation and heat retention is essentially a matter of whether minimum ventilation is implemented accurately, not whether one dares to ventilate. An ammonia detector costing a few hundred yuan is much cheaper than a batch of respiratory medicines, and much cheaper than the cost of one case of “immunization failure” against bursal disease.
The fourth line of defense: nutrition—“feeding” gut health and stress resistance
After the antibiotic ban, the gut has shifted from being “protected by medication” to having to “hold up on its own.” At the basic nutrition level, amino acid balance and stage-specific vitamin and mineral supplementation are essential. There is already fairly consistent evidence that vitamins A, C, E, and D3, along with zinc and selenium, support immune function under stress. But functional nutrition is what truly makes the difference: probiotics and prebiotics stabilize the gut microbiota; plant extracts and polyphenols alleviate oxidative stress; and enzyme preparations improve feed-ingredient utilization. These are not merely “the icing on the cake”; in the absence of antibiotics as a safety net, they reduce the immune burden on the gut.
One detail that deserves even more attention is mycotoxins. They do not directly “cause disease,” but they quietly consume the immune resources of the liver and intestines, lowering the flock’s overall tolerance threshold to various stressors. In an antibiotic-free environment, the use of mycotoxin binders should shift from “adding them after a problem arises” to “making them part of the regular feed formulation.”
Ultimately, immunity is not something you “supplement”; it comes from “leaking less.”
Genetic diseases and immunosuppressive pressures in the broader environment will not disappear just because we are anxious. But flock immunity is still largely in the hands of those managing it. In breeding-stock sourcing, avoid one more purchase of birds with an “unclear foundation”; in the environment, prevent one more instance of excessive ammonia; in the vaccination program, avoid one more instance of the “vaccinate and be done with it” routine; and in the gut, prevent one more bout of chronic depletion caused by mycotoxins. Each of these four lines of defense that is maintained gives the flock’s own immune system a little more room to keep pathogens out.
Disease-resistance progress in breeding is worth looking forward to, but it will take time to reach commercial poultry houses. Until then, managing everything that can be controlled to the fullest is the most practical form of “disease-resistant breeding.”