Broiler chickens are genetically selected to reach market weight in about six weeks, and keeping them much longer creates a cascade of problems: declining welfare, worse meat quality, and rising costs with diminishing returns. The modern broiler gains weight so rapidly that its skeleton, heart, and lungs struggle to keep pace even within that short window. Extending beyond it amplifies every one of those mismatches.
They Already Reach Market Weight by Week 6
A standard commercial broiler like the Ross 308 hits roughly 2.5 kg (about 5.5 lbs) by day 42. That weight is the sweet spot for most whole-bird and cut-up processing. By day 56 (week 8), the same bird reaches around 3.8 to 4 kg, but the extra weight comes at a steep cost in feed, health, and quality. The feed conversion ratio, which measures how efficiently a bird turns feed into body weight, worsens significantly after six weeks. The bird still grows, but each additional kilogram of meat requires substantially more feed to produce.
Fat Replaces Muscle as the Primary Gain
After about 36 days, the composition of weight gain shifts. Research measuring abdominal fat pads in broilers between days 36 and 54 found that while total body weight increased 72 to 75%, abdominal fat as a percentage of body weight climbed 23% in males and 38% in females. In practical terms, older broilers deposit proportionally more fat and less lean meat. This makes the carcass less desirable for processors and consumers alike, since excess abdominal fat is trimmed and discarded during processing.
Leg Problems Get Worse With Every Extra Day
Broiler skeletal development simply cannot match the pace of muscle growth. Even at an average age of 29 days, a large-scale study of 7,500 birds found that about 19% already showed moderate to severe lameness. Roughly 16% had gait problems obvious enough to affect their ability to move around, 2.4% could manage only a few steps, and just over half a percent couldn’t walk at all.
These numbers worsen as the birds get heavier. A broiler at week 8 carries over 50% more body weight than at week 6, all supported by the same immature leg bones and joints. Birds that can’t reach food and water lose condition quickly, and lame birds that sit in wet litter develop painful skin burns on their hocks and breast. Keeping broilers past six weeks means a larger share of the flock is effectively immobile and suffering.
Heart and Lung Failure Peaks in This Window
Sudden death syndrome (SDS) is one of the most common causes of mortality in fast-growing broilers. It typically strikes otherwise healthy-looking birds that are found dead on their backs with no prior symptoms. The underlying cause is cardiac failure: the heart grows too slowly relative to the oxygen demands of a rapidly expanding body.
Research tracking daily SDS death rates found they climb through the first three weeks and remain elevated at 80 to 86 deaths per 100,000 birds per day through day 39. The rate does gradually decline after that, but the cumulative toll keeps rising with every extra day the flock is kept. Ascites, a related condition where fluid accumulates in the abdomen because the heart can’t pump blood through the lungs efficiently, follows a similar pattern. Both conditions are directly tied to rapid growth rate, and both claim more birds the longer a flock stays on the ground.
Meat Quality Deteriorates in Older Birds
Two muscle conditions, white striping and woody breast, are increasingly common in modern broilers and get significantly worse with age. White striping appears as visible lines of fat running through the breast meat. Woody breast is exactly what it sounds like: the fillet becomes hard, pale, and rubbery, sometimes with a gritty texture that consumers reject.
A study that processed 1,920 birds at both 6 and 9 weeks of age found a clear increase in severe and very severe cases of both conditions in the older group. The heavier and thicker the breast fillet, the more likely it was to be affected. At 9 weeks, more fillets showed higher severity scores, and the relationship between fillet weight and the presence of these defects plateaued, meaning nearly all the largest fillets were compromised. These myopathies don’t just affect appearance. They change the texture, water-holding capacity, and cooking quality of the meat, making it harder to sell as a premium product.
The Economics Stop Making Sense
All of these factors converge into a straightforward economic argument. After six weeks, each day costs more feed per unit of weight gained, a growing percentage of that gain is fat rather than saleable meat, mortality from heart failure continues to accumulate, more birds develop leg problems that downgrade carcass quality at slaughter, and breast meat defects become more common. The processor ends up paying more to produce a less valuable bird.
For farmers raising birds on contract, the calculation is even simpler. Feed is typically the single largest expense, and the grower is often paid based on a formula that rewards efficient conversion. Holding birds past the target date burns through feed without a proportional return in carcass value. The six-week window is not arbitrary. It represents the point where the genetics of modern broilers, the economics of feed conversion, and the limits of the bird’s own body all intersect.