Muscle vs fat growth: Impact on FCR in broilers

Feed intake is the primary factor driving growth rate but feed efficiency is driven more by muscle than fat.
calendar icon 25 September 2023
clock icon 4 minute read

At the recent EW Nutrition Poultry Academy in Jakarta, Indonesia, Dr Steve Leeson, Professor Emeritus, University of Guelph, Canada, stressed that feed intake is the primary factor driving the growth rate of broilers, but feed efficiency is driven more by muscle deposition than by fat growth.

The selection for rapid growth has resulted in appetite indirectly becoming an important criterion in the selection of modern broiler strains. These strains are very lean. He noted that “encouraging muscle compared to fat growth always improves feed efficiency. The reason is very simple – 1kg of fat contains 9,000 kcal, whereas 1kg of muscle, which is 80% water is only 1,000 kcal. Consequently, broilers are now very responsive to amino acids. This means that depositing muscle is almost ten-times more efficient than depositing fat.”

Dr. Steve Leeson

He also commented that feeding the broiler chick is now more critical than ever. “Seven-day bodyweight is now a standard metric for measuring productivity. Each 1 gram of bodyweight at 7-days of age is equivalent to 10 grams at 35-days.”

Factors affecting feed intake

To maximise the genetic potential and take advantage of the appetite of modern broilers, it is essential to minimise the factors that suppress feed intake, which include:

  • Feed form – feed intake: mash< crumbles< pellets,
  • Feed particle size – feed intake is maximised by feeding as large a feed particle as possible,
  • Stocking density – there is unlikely to be true ad lib., feeding after 28 days of age at densities >35kg/m2,
  • Environmental temperature – maximum feed intake is around 15oC after brooding, however, optimum feed:gain is around 26oC,
  • Lighting – the longer the hours of light the greater the feed intake, however 4 hours of darkness is needed, so as not to compromise bird health and immune response,
  • Dietary energy level – broilers still eat to energy requirements.

Stocking density and feed restriction

Broilers often reach their target bodyweights at 21 days of age, but from 28 days fail to achieve their genetic potential. Often, this lag is incorrectly blamed on changing to a finisher diet, subclinical disease, or other challenges. In many cases, however, this is simply due to reduced feed intake caused by limited access to feeders. This is because at higher commercial stocking densities, that maximise profitability per house, not per bird, larger broilers commonly compete for access to feeders, and may not achieve ad lib., feed intake. Broilers need to eat for about 8 minutes every hour, although typically, this does not occur in a single feeding.

The genetic potential for feed intake (grams/day) is related to bird age. For example, for a 21–42-day broiler, feed intake is age (days) x 6, so a 28-day-old broiler can consume 168 grams feed/day.

Environmental temperature

Modern broilers are increasingly sensitive to heat stress. We can influence energy need by reducing maintenance needs for sustaining body temperature. Maintaining broilers at <15°C is going to be challenging at high stocking densities, so keeping broilers as close to a thermal neutral of around 24°C is going to minimise energy needs regardless of stocking density.

Options to maintain feed intake

Improving pellet quality

Increasing pellet quality means birds need less time to consume fee, thereby using less maintenance energy. Improving pellet quality means you have the option to reduce dietary apparent metabolisable energy. For example, if pellet quality is improved from 60% to 80%, it is equivalent to increasing dietary energy by 60kcal, without any change in diet composition. Alternatively, AME can be reduced by 60 kcal, reducing diet cost without compromising performance.

Transition from crumbles to pellets

Transition from crumbs to pellets often occurs too late commercially, and this limits growth rate. Reluctance to feed pellets earlier is due to perceived feed refusal, so the change from crumbles to pellets is often at 21-24 days. Broilers will eat large particles of feed at very young ages. This transitory feed refusal, which occurs for minutes or hours, not days. Wasted feed is overestimated and is a maximum of only 1-2 grams/bird. Feed refusal can be minimised by adding 5% pellets to the last load of crumbles and making the first load of grower feed 50% crumbles and 50% pellets.

Pellet size

Matching pellet size to bird age becomes critical as stocking density increases. Dr Leeson recommended that from the bird’s perspective ideal pellet sizes are: pre-starter (0-10 days) 2mm, starter (11-21 days) 3.5mm short, grower (22-32 days) 4mm, and finisher (32+ days) 5mm. When given a selection of feed in mixed particle sizes, birds invariably show a preference for the largest particles. As pellet size increases so the bird needs to consume fewer pellets and so spend less time at the feeder.


Dr Leeson concluded that feed intake dictates growth rate, so get birds onto pellets as soon as possible, and change the pellet to suit the bird. Any management practice that limits feed intake, such as high stocking density or heat stress, means that diet changes should be correspondingly delayed.

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