An S1 strain is the first generation of seeds produced when a plant pollinates itself, a process breeders call “selfing.” The “S” stands for “selfed” and the “1” marks the first generation. This technique is most widely used in cannabis breeding, where growers force a female plant to produce pollen and fertilize itself (or a clone of itself), creating seeds that carry genetics from only one parent.
How Selfing Works
Plants normally reproduce by combining genetic material from two parents. Selfing bypasses that entirely. A female plant is chemically triggered to grow male pollen sacs, and that pollen is then used to fertilize the same plant or an identical clone. Because both the “mother” and “father” are genetically the same individual, the resulting S1 seeds are far more uniform than a typical cross between two different plants.
The most common way to trigger this is by spraying a female plant with silver thiosulfate (STS) or colloidal silver solution. These chemicals suppress the plant’s natural female flowering hormones, causing it to develop male pollen structures instead. You typically begin spraying a few days before switching the plant to its flowering light cycle and continue until pollen sacs form, usually a couple of weeks into flowering. The pollen then lands on unfertilized female flowers on the same plant or its clone, and seeds develop normally from there.
STS is generally considered more reliable and easier to work with than colloidal silver, producing a higher success rate with fewer applications. Pre-mixed STS kits are widely available from breeding supply companies.
Why Breeders Create S1 Seeds
The primary reason to make S1 seeds is to preserve a specific plant’s traits without introducing outside genetics. If you have a female plant with exceptional flavor, potency, or growth characteristics, selfing lets you reproduce those genetics in seed form rather than relying solely on clones. Every S1 seed is female, since the genetics come entirely from a female parent with no male chromosomes in the mix.
S1 seeds also give breeders a way to “bank” genetics long term. Clones degrade over many generations and require constant maintenance, while seeds can be stored for years. For home growers who find a standout plant in a pack of seeds, selfing is often the simplest path to keeping those genetics alive.
S1 vs. F1: Key Differences
F1 seeds come from crossing two distinct parent plants. The offspring combine genetics from both parents, which often produces what breeders call “hybrid vigor,” where the plants grow more vigorously than either parent alone. However, F1 seeds can show a wider range of traits because they’re pulling from two separate gene pools.
S1 seeds draw from a single genetic source, so they tend to produce plants that look and behave more like the original mother. That said, S1 seeds aren’t true clones. The selfing process can reveal hidden recessive traits that were masked in the parent, so you may still see some variation among S1 plants. This variation is usually less dramatic than what you’d see in an F2 generation (the offspring of two F1 plants), but it’s worth knowing that not every S1 seed will be an exact copy of the mother.
S1 in Other Contexts: Livestock Vaccination
Outside of plant breeding, “S19 strain” (sometimes shortened to S19) refers to something completely different: a weakened form of the bacterium Brucella abortus used to vaccinate cattle against brucellosis, a disease that causes miscarriages in livestock and can spread to humans. Dr. John Buck first isolated this naturally weakened strain from the milk of American Jersey cattle in 1923, and it was used worldwide as the standard cattle vaccine from the early 1930s until it was largely replaced by a newer vaccine strain called RB51 in the 1990s.
The S19 vaccine works by triggering a strong immune response without causing full-blown disease. Vaccinated cattle develop protective antibodies within the first three weeks, and their immune systems shift toward a type of defense that activates infection-fighting white blood cells, the same kind of response needed to clear Brucella from the body. A systematic review of 33 field trials found that about 61% showed significantly lower infection rates in vaccinated groups, though the vaccine works best when combined with other control measures like testing and culling infected animals.
One practical concern with S19 is that it can infect humans through accidental needle sticks or contact with mucous membranes. A survey of Canadian veterinarians found that 46% had accidentally injected themselves at least once while vaccinating cattle, and roughly one in five of those developed moderate to severe symptoms including fever, fatigue, joint stiffness, headaches, and muscle pain. Reported cases have generally resolved with antibiotic treatment lasting six to eight weeks, and no fatal or permanent injuries have been documented from occupational exposure to any of the licensed Brucella vaccine strains.