In tropical and subtropical countries, the layers are subjected to important heat stress. This heat stress can occur during long periods. The lower feed intake noticed during these periods is the result of the bird’s reduced ability to lose heat. Pullets and laying hens should maintain their body temperature constant by adjusting their food intake according to their ability to lose heat.

The lower growth rates during rearing and the reduced production during lay are only consequences of the reduction in feed consumption when the birds are incapable of regulating their temperature.

The influence of heat and the behaviour of laying hens

A better understanding of the mechanisms of thermoregulation helps us to reduce the impact of heat on growth and production.

- To evacuate extra heat, the bird will increase the heat loss by direct exchange with the immediate exterior environment (air, litter, and radiation). As the environmental temperature becomes more and more difficult and as the upper critical temperature is reached, the body temperature starts to increase.

- Heat elimination is then brought about by respiratory means. Heat loss is reduced as the humidity is increased. A relative humidity above 70 % renders thermoregulation very difficult in hot humid conditions.

- Energy and protein synthesis decrease as soon as the body temperature increases. The production of extra heat during the process of digestion lowers the upper critical temperature, increases body temperature and reduces the resistance to heat.

When the conditions are too severe, there will be a reduction in the quantity of food consumed.

Birds have to match their feed intake to their ability to eliminate the heat produced.

The energy intake is the limiting factor for growth and production.

- The dietary energy level has no effect on growth, thus showing that thermoregulation is responsible for the lower growth.

- During the laying period, energy consumption is not modified by changing the energy level of the diet. Growth and production are reduced at the same time as temperature is increased.- As indication only, because air movement and humidity have effects on thermoregulation:

o Egg weight falls by about 0.4% per °C between 23 and 27°C; above 27°C the decrease is about 0.8% per °C.

o Growth at point of lay is reduced above 24°C, and is extremely low above 28°C.

o Rate of lay is generally only affected above 30°C.

o Feed conversion ratio is minimum at a temperature of about 28°C; above 28°C it increases, because of the fall in production.

The quantity of heat to be lost by convection increases dramatically with increases in ambient temperature. If that is not possible, the mechanisms of thermoregulation come into play gradually with effects on feed consumption.

At high temperatures, the food consumption and production of fully feathered birds are generally lower than of those which are only 50% feathered. This shows that in hot temperatures it is the capability to lose the heat produced, which limits production. As the temperature increases, the thermoregulatory mechanisms, spreading the wings, acceleration of cardiac and respiratory rhythms, are gradually set in motion.

How to reduce the adverse effects of high temperature on growth and laying performances?

As the first effect of high temperature is to increase the heat loss by convection and then via the respiratory route, it is important to increase heat loss:

- By reducing ambient temperature when it is possible (insulation of the roof, reduce the penetration of sunrays in the house, etc…).
- By facilitating heat loss through convection (cages design, …).
- By increasing speed of air movement.
- By cooling the air.
- By providing a cool water: Water is an important tool for the chicks to evacuate part of the heat. The cooler the water the better the birds can tolerate high temperatures.

Providing a cool water to the birds will increase their consumption and reduce the quantity of energy to be removed (1,5 % for a reduction of 10 °C of water temperature).

Both in rearing and in production, growth and laying performances can only be maintained by facilitating heat loss and/or by allowing the birds to consume their food the cooler part of the day at which time they can more easily lose the specific heat of digestion (extra heat).

In rearing period:

The growth is affected when the birds approach full plumage. It is therefore important to arrange it so that excellent growth and uniformity is obtained during the first 5 weeks.

- One extra feeder and one extra open drinker compared to normal conditions have to be available.

- The growth during the first weeks of age is also related to light duration. So, a slow step down lighting programme will help to boost feed consumption and growth. The reduction of light duration done in the evening will allow the birds to eat early in the morning during the cooler part of the day and help the bird to lose easily the specific heat of digestion before hotter part of the day.

- As the pullets are not able to adapt their feed intake to the energy level during the first 5 weeks, a starter feed rich in energy and in protein will help the growth. A starter feed in crumbs during this period will encourage feed intake and growth.

After 6 weeks:

- As 25 % of the maintenance requirement is devoted to eating the food, a uniform texture  75 % of the particles between 0,5 and 3,2 mm) will reduce the energy needed.

- Giving the feed early in the afternoon will encourage feed consumption of large particles size (low energy needed) before the light off. Fine particles will be easily eaten in the morning. This meal feeding will encourage the development of the digestive tract and help the increase of consumption at start of lay.

- If the birds are not obtaining the bodyweight at a given time, one should be delaying the change of feed (starter to grower and grower to developer).

A too low bodyweight at start of lay will lead to post peak dips, risks of high mortality in production with some prolapse, and poor quality and persistency later on in production.

- A too early light stimulation will lead to post peak dips. No light stimulation is necessary before 2 % of lay. Light increase is managed to meet increase of production.

- In order to obtain pullets with enough reserves, a pre lay feed during 2 weeks before 2 % of lay with 2 % of Ca is advised. The addition calcium should be in particles to avoid negative effect of powder on feed consumption.

In production period:

- As during the rearing period, the impact of high temperature will be reduced if feed intake is encouraged during the cooler part of the day. All increase of light duration after 2 % of lay is advised in the morning.

- 2 hours of light from 2 % of production can be gradually placed in the middle of the night to encourage feed intake (so growth) and to provide Calcium during the intensive shell deposit.

- Giving lights from 4 to 20 hours and at night from 23 hours to 1 o'clock in the morning appears to be the programme adopted in most hot countries, between latitudes 20° north and 20° south.

Feed during production period:

- To avoid any under-consumption at start of lay, the use of a similar energy level in production as during the second part of the rearing period will not penalize feed intake.

Energy level during production period should be constant throughout the laying period.

- Addition of fats during the laying period has some advantages: the feed becomes more palatable with a lower heat increment. Shell thickness and egg weight seems to be improved. Production is maintained and feed efficiency improved.

- 75 to 80 % of the amino acids assimilated by a chicken are used directly for the production of egg material. So, amino acids requirements for optimum production have to be covered. Amino acids content in the feed should be adapted to real feed consumption noticed.

- The attractiveness of a feed depends to a large extent on its texture. In theory, presenting food in crumb or pellet form, will lead to an increase in food consumption, provided that the technology and the raw materials used give the hens access to a good quality crumb.

- In a hot climate, a diet with good texture help to reduce the degree of under consumption observed in summer conditions. For this reason, we advise having a diet with a minimum of 75 to 80 % of the particles between 0.5 and 32 mm diameter.

Management of egg quality in hot climate:

These previous tools will help to improve the growth and the feed consumption of the future layers and layers. This should lead to an improvement of the production and the liveability. To optimise the egg quality, it is also important to take care of the quantity of calcium in the diet according to the age, the size of the calcium provided.

To limit the natural deterioration of the egg quality throughout the laying period, four majors points have to be it is important to take into account the following aspects:

- To insure that the bird has developed good reserves of calcium before the first egg
- To insure that calcium is available during the shell formation
- To provide a optimum size of limestone during the shell formation period
- To adapt quantity of limestone according to the age

The obtention of the right bodyweight associated to the utilisation of a pre lay feed the 10 days before the first egg will encourage the development of the calcium reserves in the medulary bone and therefore the egg quality during the second part of the laying period. The prelay feed is an intermediate feed between developer and layer feed. The calcium content should be 2% with 50 % of this amount in particles (from 2 to 4 mm).

The intensive deposit of calcium occurs during the last part of the day and during the night. Calcification is completed for 80 % of the birds 2 to 3 hours after light on. Higher will be the feed intake and the calcium intake in the afternoon, higher will be the calcium deposit and therefore stronger should be the egg shell. An adapted feeding timetable reduces the mobilisation of the calcium from the medulary bone. The introduction of a period of light in the middle of the night improves the shell quality by allowing the hen to replenish her calcium reserves.

Eggshell quality depends to a large extent on the quantity of calcium remaining in the gizzard at the end of the calcification process and, therefore towards the end of the night. By using a limestone with a particle size between 2 and 4 mm, say 2 – 4.5 mm, the hen has calcium available at the end of the night, thanks to a progressive solubilisation of the calcium.

The increase in the level of calcium in the feed is justified by the increase in the weight of the egg shell and the fact that calcium from medulary bone is less available during the second part of lay.

Conclusion:

The harmful effects of heat, which affect both growth and production, can be reduced considerably by recognising the physiological requirements of the birds.

The conception of the laying house, the water temperature provided, the lighting programmes, the feeding timetables, the feed content and presentation are additional measures which allow to reduce the problems cause by high temperatures. But the results obtained depend to a large extent on diligent everyday care of the birds.

References:

Bougon (1996) Report to Itavi Seminar1996

Buyse et al (1993) British Poultry Science 34 : 699 – 709

Gabarou et al (1998) British Poultry Science 39 : 79 - 89

Grizzle et al (1992) British Poultry Science 33 : 781 - 794

Leeson et al (1997) Commercial Poultry Nutrition 2nd Edition

Lewis et al (1996) British Poultry Science 37 : 279 - 293

Morris (1995) British Poultry Science 36 : 763 - 769

Morris (1967) Environmental Control in Poultry Production. Carter (Ed.):15–39

Xin et al (2002) British Poultry Science 81 : 608 – 617

Zollitsch et al (1996) International Symposium on Requirements of Poultry and Swine. Ed. Rostagno Joly (2003) ISA technical bulletin