Broiler Production

Introduction

Since the start of the modern broiler industry, feed efficiency has been one of the major criteria used in defining the performance of broiler chickens. In North America, feed efficiency is described as feed intake:weight gain. In Europe, the calculation is usually reversed, being described as weight gain:feed intake. What ever system is used, the idea is to obtain a measure of how efficiently the feed is being utilized, which is obviously of economical importance because feed represents about 65% of total cost of production. Feed is used by the bird for two basic reasons, namely for growth and for maintenance. In young birds most feed is used for growth and little is used for maintenance, and so efficiency is very good. Over time efficiency deteriorates because the broiler has an ever-increasing body mass to maintain. Over the years we have seen a steady decline in feed efficiency from around 2.2 in the early 1960's to 1.75 today under certain situations. This continually improving situation is due to improved genetic potential, and the fact that more feed is directed towards growth (and less for maintenance) as days to market decline. However many in the poultry meat business are now questioning the usefulness of classical feed efficiency. The main reason for such concern relates to changes in the industry, where today we have a vast range of potential bird weights and ages, together with males and females potentially being grown separately, and so consequently there is a range of dietary specifications and feeding programs. Today, there is a trend to consideration of energy efficiency rather than feed efficiency, and for new measures of efficiency, such as feed cost/kg live weight through to feed cost/kg deboned breast meat or even feed cost per kg meat per square meter of floor space. Following is a review of factors that can influence feed efficiency, together with discussion on some new ideas for quantitating feed usage in relation to limitations of such classical values.

Diet energy level

It has been suggested that the broiler eats to its maximum physical capacity, and varying the energy density of the diet can easily control the birds' energy intake. 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 boiler gets older it does seem to adjust its intake in relation to diet energy level so as to normalize its energy intake. Table 1 shows the results of diluting the feed to very low levels in diets for birds from 35-49d.

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

Adapted from Leeson et al. (1996)

In this study, a series of diets were produced by diluting a conventional 3200 kcal/kg finisher, by adding oat hulls. As the diet was diluted, the birds simply ate proportionally more feed, and body weight was little affected. It is surprising that the broiler can grow adequately on a diet containing just 1600 kcal ME/kg and 9% crude protein. As shown in Table 1, the bird was able to maintain growth simply by eating vast quantities of feed. Classical feed efficiency obviously declined, and with the most dilute diet the feed:gain was 4.01 from 35-49d. Most producers could be very concerned with this number. However, if we look at energy efficiency, the broilers fed this very dilute diet, in fact, converted energy most efficiently. In fact, as classical feed efficiency declined, so energy efficiency improved. Obviously it is totally impractical to feed such extremely diluted diets, but the study does show that classical feed efficiency in this situaiton was very misleading in defining energy efficiency (or energy costs).

High energy feeds obviously cost much more than do low energy feeds, and so there is a price to pay for improved feed efficiency brought about by using, for example, high-fat diets. Table 2 shows an example of using diets with a mean energy value of from 3000 up to 3300 kcal/kg fed to male broilers to 45d. Diet energy has no effect on growth rate, but will affect feed intake as discussed previously. Feed:gain therefore improves as diet energy level increases. However, the higher energy feeds are more costly, and when we calculate feed cost per bird, the least cost in this particular case is seen at 31000 kcal ME/kg. As often happened, in this situaiton, the best feed efficiency was not the most profitable. Obviously economic evaluations must be calculated for local conditions.

TABLE 2. Effect of diet energy level on broiler performance and feed costs.

So called 'broiler growth models' today are able to identify the most profitable diet, given feed prices, broiler prices, expected performance, etc. The diet is ultimately least-costed in the traditional way, but this prior selection is often referred to as "maximum profit formulation".

Separate-sex growout

The feed efficiency of female broilers will usually be higher (less efficient) than for 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 (Leeson et al., 1988). Body fat takes 9 times as much feed energy to produce as does muscle. 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. 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 every-increasing body mass. Consequently, feed efficiency deteriorates over time.

Barn temperature

The broilers' maintenance energy needs are greatly influenced by the temperature of its environment. After initial brooding, the bird must use some of its feed to maintain its body temperature (Sandercock et al., 1995). Under ideal conditions of around 20-25°C, 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°C below 20°C. Between 20-25°C, the bird will eat about 1% less per 1°C increase in temperature, and so here feed efficiency will improve (May et al., 1998).

Above 25°C (depending upon acclimatization), hat stress conditions can occur, and then feed efficiency will again deteriorate because now the bird is using energy to stay cool (panting, etc.). Under these conditions, feed efficiency deteriorates further because the bird is reluctant to eat feed and so proportionally more feed is directed towards maintenance, and less can be used for growth. (Al Harthi and Macleod, 1996).

Health management

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 (Brandon et al., 1997). 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, 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 consequence of microbial challenge.

New measures of feed efficiency

Following are some new measures of feed efficiency and factors relating to their use: 

Summary

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 world-wide 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 profitable, because economics can dictate the optimum use of lower rather than higher, diet energy levels. A more useful measure of feed usage today is energy intake per unit of weight gain. For male birds the goals are for around 6.2 Mcals metabolizable energy per kg weight gain for 7 week-old birds. In the future we may consider feed costs per unit of meat or may even have to take into account efficiency of barn usage and also manure output as they influence environmental stewardship.

Dr. Steve Leeson
Department of Animal & Poultry Science
University of Guelph, Guelph, Ontario Canada N1G 2W12

References

Al-Harthi, M.A. and M.G. Macleod, 1996. Analysis of heat stress effects on growth by pair feeding. Br. Poultry Sci. 37:542-43.

Branton, S.L., B.D. Lott, J.W. Deaton, W.R. Maslin, F.W. Hustin, L.M. Pote, R.W. Keirs, M.A.

Latour, and E.J. Day, 1997. The effect of added complex carbohydrates or added dietary fiber on necrotic enteritis lesions in broiler chickens. Poultry Sci. 76:24-78.

Leeson, S., L.J. Caston and J.D. Summers, 1988. Response of male and female broilers to diet protein. Can. J. Anim. Sci. 68:881-890.

Leeson, S., L.J. Caston and J.D. Summers, 1996. Broiler response to diet energy. Poultry Sci. 75:529-535.

May, J.D., B.D. Lott and J.D. Simmons, 1998. The effect of environmental temperature and body weight on growth rate and feed:gain of male broilers. Poultry Sci. 77:499-501.

Sandercock, D.A., M.A. Mitchell and M.G. Macleod, 1995. Metabolic heat produciton in fast and slow growing broiler chickens during acute heat stress. Poultry Sci., 36;868.