Together with growth rate, days to market and mortality, feed efficiency has been considered as one of the important parameters in assessing the potential of bird strain or feeding program etc. In N. America the value is calculated by dividing feed intake by weight gain, and so values of around 1.9 are common for 42 d old birds. In some European countries, the efficiency is calculated as weight gain divided by feed intake, and a corresponding value would be 0.53. Whatever system is used, measures of feed efficiency are useful in describing feed intake in relation to growth rate. Feed efficiency is, therefore, a useful measure of performance as long as all other factors affecting both growth and feed intake are either minor or do not vary from flock to flock.

Today, we have many factors affecting both growth rate and feed intake, because we have now moved from standardized growing programs to one tailored to meet specific local goals and economic conditions. The single largest factor affecting feed efficiency is energy level of the feed. Five to ten years ago, this was not a major concern because most broilers were fed on diets containing around 3000 kcal/kg in the starter, up to 3200-3300 kcal/kg in the finisher. Now because of high energy prices, and other management problems, we often see much lower energy values used in one or all diets of a feeding program, and so it is now more difficult to pin-point a standard energy level in the feed. We are also growing broiler chickens over a much more variable time frame, and this also affects feed efficiency. For example feed efficiency in a 60d roaster male is expected to be higher than for a 35 d female destined for the cut-up trade. Similarly we now have broilers grown in most countries of the world, and so environmental temperature will affect maintenance energy need, and hence classical feed efficiency. These factors now mean that feed efficiency can be quite a variable number, and as such is perhaps losing its significance in being able to compare broiler performance under a range of field conditions. Following is a more detailed review of these factors affecting feed efficiency.

Diet Energy Level

It seems as though the broiler chicken is still eating to its energy requirement. It has been suggested that the bird eats to its maximum physical capacity, and that the birds' energy intake can easily be controlled by varying the energy density of the diet. This fact may be true to some extent with the young broiler, because we can temper early growth rate (ascites control programs, for example) by feeding lower energy diets. However as the broiler gets older it does seem to adjust its intake in relation to diet energy level. Table 1 shows the results of diluting the feed to very low levels.

Table 1. Effect of diet dilution from 35-49d of age on broiler performance.

Diet ME
Diet CP
49d body wt
Feed intake
35-49d (g)
Energy efficiency
(Mcal/kg gain)

Adapted from Leeson et al. (1996)

As the nutrient level of the diet was reduced, so birds ate more feed. This means that the bird is not eating to physical capacity, because the bird was able to almost double its normal intake on the very low nutrient dense diet. This amazing ability to adjust feed intake resulted in no real difference in 49d body weight. As the birds eat more feed at constant growth rate, then feed efficiency starts to deteriorate. A feed efficiency of 4.01 from 35-49d would hardly seem to be economical. However, if we calculate energy efficiency, then the birds on the lowest energy feed were actually the most efficient in converting feed energy to weight gain. This is a good example of classical measures of feed efficiency being totally misleading. It is unlikely that the low energy levels used in Table 1 would be economical, because it is difficult to find low energy ingredients that are inexpensive per unit of energy. However these data do show that we can consider a range of energy levels for the broiler, without affecting growth rate too much, and so diet choice is simply a matter of allowing our formulation programs to select the most optimum solution.

Male vs Female Birds

The feed efficiency of female broilers will usually be higher (less efficient) than male birds of corresponding weight, after about 30 days of age. The reason for this is that female birds tend to deposit proportionally more fat in the carcass. Body fat takes 9 times as much feed energy to produce as does muscle. The reasons for this is that fat contains more energy than does protein per unit of weight, and more importantly, muscle is only about 20% protein by weight, the remainder being water. For this reason it is usually uneconomical to grow female broilers much beyond 45d unless special emphasis is placed on reducing fat deposition . Likewise with heavy male birds, feed efficiency is going to be greatly influenced by the growth of fat vs muscle.

Bird Age

As birds get older, their feed efficiency will deteriorate. This situation is simply due to the fact that heavy birds use increasing quantities of feed to maintain their body mass, and less is used for growth. In the 7d old bird, about 80% of feed is directed to growth and only 20% is needed to maintain the small body size - consequently feed is used very efficiently. In an 8-week old bird these numbers are reversed such that only 20% of feed is used for growth, and 80% is needed to maintain the ever-increasing body mass - feed efficiency, therefore, deteriorates.

Environmental Temperature

The broilers' maintenance needs are greatly influenced by the temperature of its environments. After initial brooding, the bird must use some of its feed to maintain its body temperature. Under ideal conditions of around 20-25 degrees Celsius, the bird uses a minimum of feed to maintain body temperature. In cooler conditions, more diet energy must be used to maintain body heat, (and so less feed is used for growth) and consequently feed efficiency will deteriorate. Feed intake will increase by about 1% for each 1 degrees Celsius below 20 degrees Celsius. Between 20-25 degrees Celsius, the bird will eat about 1% less per 1 degrees Celsius increase in temperature, and so here feed efficiency will improve. Above 25 degrees Celsius (depending upon acclimatization), heat stress conditions can occur, and here feed efficiency will again deteriorate because now the bird is using energy to stay cool (panting, etc.). Under these conditions, efficiency of feed further deteriorates because the bird is reluctant to eat feed, and so proportionally more feed is directed towards maintenance, and less can be used for growth.

Bird Health

Obviously an unhealthy bird is likely to have poor feed efficiency. The main reason for this is that feed intake is reduced, and so again proportionally more feed is directed towards maintenance. With enteric diseases there can be more subtle changes in feed utilization because various parasites and microbes can reduce the efficiency of digestion and absorption of nutrients. A bird with sub-clinical coccidiosis is not likely to absorb nutrients with optimum efficiency, because the oocytes will destroy some of the cells lining the gut. More recently the phenomena of so-called "feed passage" has been observed in broilers. Undigested feed particles are seen in the excreta, and so consequently feed efficiency will be affected. The exact cause of this problem is unknown, but is most likely the consequences of microbial challenge.

Other Measures of Feed Efficiency

The previous discussion suggests that feed efficiency is a moving target, and today striving for a low numerical value for feed efficiency may not always be the most economical situation. A much more useful measure will be feed cost/kg weight gain, or some further variation of this such as cost/kg deboned meat, etc. A very useful starting point in re-evaluating efficiency of feed use is to consider conversion of feed energy to liveweight gain. Following are typical energy conversion figures for broilers up to 9 weeks of age (Table 2).

Table 2. Energy conversion to live weight for broilers (Mcal metabolizable energy/kg live weight gain).

of age
Male birds
Female birds
Mixed sex


Feed efficiency of broilers is affected by bird age, sex, health and environmental temperature, although the major factor is usually diet energy concentration. With a very wide range of diet energy concentrations used worldwide today, classical measures of feed intake:weight gain (or weight gain:feed intake) become less meaningful. The "lowest" feed efficiency may not always be the most economical, because economics may dictate the optimum use of low rather than high diet energy levels. A more useful measure of feed usage is energy intake per unit of weight gain. For male birds the goals are for 6.2 Mcals metabolizable energy per kg weight gain for 6 week-old birds.

For more information:
Toll Free: 1-877-424-1300