Switch from grower to layer feed when your pullets reach 16 to 18 weeks of age, or when you spot the first egg in the nesting box, whichever comes first. The timing matters because the key difference between these feeds isn’t protein (both contain 16–18%) but calcium. Layer feed is formulated with significantly more calcium to support eggshell production, and introducing that extra calcium too early can damage young birds’ kidneys.
Why 16 to 18 Weeks Is the Target Window
Around week 16, a pullet’s body begins a dramatic internal shift from growth mode to reproductive maturity. Her ovarian follicles start producing estrogen, which triggers a process called medullary bone formation. This is the creation of a specialized calcium reservoir inside the hollow cavities of her long bones, particularly the femur and tibia. That reservoir acts like a biological savings account: calcium gets deposited and then rapidly withdrawn each time the hen forms an eggshell.
This transition period from roughly week 16 to week 24 is characterized by a steady rise in blood calcium levels and increasing estrogen. Research published in Poultry Science tracked the process closely and found that plasma estrogen levels began climbing at weeks 20 to 21, with the medullary bone calcium stores not fully mineralizing until around week 23. By that point, 80 to 95% of hens in the study had already begun laying. So the body needs dietary calcium available well before the internal calcium machinery is fully built out.
Starting the transition at 16 to 17 weeks gives your birds access to that calcium right as their bodies begin demanding it, without forcing it on them weeks too early.
What Happens If You Switch Too Early
Young pullets that haven’t begun the hormonal shift toward laying simply can’t process the high calcium content in layer feed. Their kidneys bear the burden. Research on growing pullets fed high-calcium diets has documented kidney injury and changes to the parathyroid glands, which regulate calcium balance in the body. In severe cases, excess calcium can lead to visceral gout, a painful condition where uric acid crystals accumulate in the organs.
The risk is highest when chicks or very young pullets (under 14 weeks) are given layer feed. If your birds are at 15 or 16 weeks and you’re unsure, it’s safer to stay on grower feed for another week than to switch too early. A few extra days on grower won’t hurt them. A few extra weeks on layer feed before their bodies are ready could.
Physical Signs Your Pullets Are Getting Close
Age is the primary guide, but your birds will also give you visible signals that they’re approaching lay. According to the University of Wisconsin Extension, a pullet nearing sexual maturity will develop a large, bright red, glossy comb and wattles that feel velvety soft and warm to the touch. A bird that isn’t close to laying has a shrunken, dull comb that feels rough and cool.
You can also check the abdomen. A hen approaching lay will have a wide, soft, expanded abdomen with thin, flexible pelvic bones. If you gently feel the space between the pelvic bones, a laying-ready hen will have roughly three finger-widths of space, compared to one finger-width in a bird that’s still weeks away. A non-laying pullet’s abdomen feels narrow, hard, and contracted.
These signs can help you make the call when you have a mixed-age flock or when you’re unsure of your birds’ exact hatch date.
Breed Differences in Timing
Not every chicken matures on the same schedule. Fast-maturing breeds like Leghorns and Golden Comets often begin laying as early as 18 weeks, so starting their feed transition at 16 weeks makes sense. Heritage breeds like Orpingtons, Brahmas, and Marans tend to take their time and may not produce a first egg until 22 to 24 weeks or even later.
For late-maturing breeds, you can start offering layer feed at 18 weeks and watch for the physical signs described above. If your birds still have small, pale combs and narrow abdomens at 18 weeks, there’s no harm in continuing grower feed with a dish of oyster shell on the side. This lets any early layers in the flock get the calcium they need without forcing it on birds that aren’t ready.
How to Make the Transition
Don’t swap feeds overnight. A sudden change can cause digestive upset and may temporarily reduce feed intake at a time when your birds need consistent nutrition. Instead, blend the two feeds over seven to ten days. Start with roughly 75% grower and 25% layer feed for the first few days, then shift to a 50/50 mix, then 25% grower and 75% layer, and finally full layer feed. This gradual approach lets the gut microbiome adjust to the new formulation.
During this window, make sure fresh water is always available. Calcium metabolism requires adequate hydration, and birds transitioning to lay will naturally increase their water intake.
The Oyster Shell Option for Mixed Flocks
If you keep a mixed flock with birds of different ages, switching the entire flock to layer feed at once creates a problem: the younger birds get too much calcium while the older ones may not get enough soon enough. The simplest solution is to keep the whole flock on grower or an all-flock feed and offer crushed oyster shell in a separate dish.
Hens approaching lay will instinctively seek out the extra calcium and eat what they need. Younger birds will largely ignore it. This self-selection method is widely used by backyard flock owners and avoids the kidney risks of feeding layer-level calcium to immature birds. Once all your birds are laying consistently, you can switch the entire flock to a standard layer ration.