By consultant nutritionist John Portsmouth
As day-length lengthens, ambient temperature almost always increases and as it does so, feed intake decreases. This is a direct result of a change in the body’s need for energy for maintenance purposes. How much feed consumption falls is directly proportionate to the rise in temperature. So if our ’normal’ laying ration is based on an ambient temperature of about 21ºC and it generally is, then if ambient temperature rises to 25-26ºC then a proportionate adjustment is theoretically necessary to compensate for the decline in feed intake.
In the Southern Mediterranean and many Middle Eastern countries, the changes in temperatures are very predictable and different ration formulations are frequently used for the different seasons. For example, the spring and late autumn seasons are not dissimilar and can be dealt with in like manner whilst winter and summer conditions are invariably markedly different. Therefore, given a temperature graph based on averages over about 3-5 years provides the information necessary to calculate the dietary differences.
Unpredictable temperatures
In the UK our temperature patterns are far from predictable and whilst an average for May to October in Southern England may be 18ºC, it could be 2-3ºC lower in Northern counties. But of course it can average 26ºC in July-August with a few odd days rising into the low 30s. In general terms however, it is the sudden rise in temperatures that causes the bird most problems. This is so-called heat stress. Of lesser concern is a gradual change such as usually occurs from spring to high summer.
The decline in energy requirement at high temperatures not only reduces calorie intake but also of course all the other important macro and micro nutrients such as amino acids, essential fatty acids, vitamins and minerals. And it is their daily reduction which cause egg mass and shell quality to decline. Keeping the energy level of the diet constant the non-energy nutrients should be increased by 1.25% for every 1ºC rise. Therefore, if during August the average temperature is 26ºC, that is 5ºC above the normal 21ºC stated earlier, then nutrients such as methionine, cystine and lysine for example should be increased by 6.25% (1.25% x 5ºC).
If the methionine level of the diet is 0.35% then to cover the anticipated decline in feed intake it should be increased to 0.37%. Obviously the factor of 6.25% should be applied to the whole diet specification, with the exception of energy.
Price premium
It could be sensibly argued that many of the vitamins and trace elements already have a safety margin much greater than 6.25% and adjustment to their level is probably not warranted. From a practical aspect, formulating a ’hot summer’ diet presents no problem although the logistics of manufacture, delivery etc may place it into the ’special’ category where it attracts a price premium above and beyond the actual diet cost. One solution is to anticipate the temperature increase in say, July-August and have the ration specification placed on standby so it can be called upon in an emergency and at an agreed premium. Obviously the producer mixing his own feeds is in the best position to react to changing environmental conditions.
Feed availability
But what else can be done to minimise the heat stress effect? Water consumption should be encouraged by ensuring that the supply is both cool and clean and that additional space is provided in extreme cases.
If it is possible, limit the feed availability to the early morning and late evening when temperatures are usually lower than those between 11.00 and 16.00 hours. Another tip is to provide a mixture of both mash and pellets (75% - 25%) to encourage consumption. During heat stress minerals are more rapidly depleted and the use of an electrolyte mix will help to prevent excess mineral tissue depletion as also will a multi-vitamin solution. Since the retention of B-complex vitamins is generally poor, the need to replenish on a daily basis is important. Use a solution that has a base material which discourages algae formation in the water system.
Global Warming
If global warming increases at the rate some scientists are predicting, then we can expect more prolonged hot summer temperatures probably on a par with Southern Europe. In such circumstances, changing the proportion of calories consumed from carbohydrates to calories from oil is a recommended additional diet adjustment.
Because oils have about 2¼ times the concentration of energy (weight for weight) compared to cereal carbohydrate, the energy used in digesting the oil is lower. This interchange of cereal energy for more oil energy reduces the amount of energy needed in digestion and thus lowers the heat increment thereby making the bird less uncomfortable during high temperatures. Generally, the amount of oil which can safely be used is governed by the physical and flow properties of the ration and less by the desirable nutritional manipulation. This dietary manipulation is more suitable to higher ambient temperatures than those currently experienced but should UK summers regularly reach into the mid 30ºC it could well be one of the important necessary nutritional adjustments.
Humidity
A complicating issue is, however, that of humidity. Its interaction with temperature can reduce feed intake even further. For example, a rise of 10-15% points in relative humidity (RH), say from 65% to 80%, may reduce feed consumption by an additional 5% or more. Whilst this effect is considerably more damaging at the higher temperatures (greater than 33ºC) it also has an added damaging effect in the mid to high 20ºC range. In the example given earlier for methionine (the same arithmetic applicable to other nutrients), the combined RH/ºC could well raise the ration methionine from 0.37% to 0.39%.
In conclusion, I would recommend that feed suppliers and home mixers draw up a hot weather stress programme consisting of diets which can be made available at short notice. One further recommendation is not to keep bulk bins too full during the critical summer months so in the event of a sudden heat wave the hot weather diet can be made more quickly available to the birds. Additionally, metal bulk bins and their contents increase in temperature during the summer months which increases the risk of oxidative rancidity occurring to the detriment of feed quality and bird performance. The frequent delivery of fresh batches of feed reduces this risk.
