How the lighting step-up can be switched off
The traditional pattern of increasing day length for pullets, followed by egg producers for years, can be simply abandoned. That was just one of the striking assertions presented to delegates at the free range conference by lighting expert Peter Lewis in his presentation 'Light, more to it than meets the eye'.
Dr Lewis also raised eyebrows among delegates by telling them that day length changes after 18 weeks have no affect at all, that laying hens 'see' light through the tops of their heads and that pale eggs are caused not by sunlight, as has been believed for years, but by heat. The key 'flashpoints' were:
A sixteen hour day in one step
Pullets arriving at a laying unit can be switched directly from 10 hours light to 16 hours, said Dr Lewis. "Total egg output in the laying year for pullets that are given a single abrupt increase in daylength will be remarkably similar to those given a series of smaller increases," he said. "The main difference will be that an abrupt change tends to produce a higher peak while a step-up programme results in a lower peak but better persistency." The effect will be to increase total egg numbers to 72 weeks (by 3 eggs) and to decrease egg weight by 0.5 gm.
In fact, went on Dr Lewis, the impact of daylength on egg numbers ceases to have an effect after 14 hours. "Egg numbers to 72 weeks increase by about four eggs for each extra hour of daylength in the laying period," he explained, "but only up to about 14 hours beyond which there is no further increase." The reason the free range industry operates on a 16 hour day is to ensure that the length of the birds' day does not decrease dramatically between summer and winter.
How the top of a hen's head lets light in
The light that birds need in order to see enters their brain through the eye. But most of the light that induces a sexual response, delegates heard, enters the top of the head and passes through feathers, skull and tissue into a part of the brain called the hypothalamus where it stimulates the release of hormones which control maturation and ovulation. Which is why blind birds reach sexual maturity and lay reasonably well. Light using the same route also stimulates the release of hormones which allow the bird to operate its internal clock and calendar.
Why flocks are laying earlier in the day
The production of an egg is stimulated by the change from light to darkness at the end of the day, Dr Lewis explained. "Roughly speaking egg-laying occurs thirty minutes later for each one hour increase in daylength," he went on. "One consequence of the selection for egg numbers (caused by a shorter egg formation time) is that each generation of modern hybrids lays its eggs about ten minutes earlier. As a result birds on 16 hour days now lay some of their eggs before the lights come on."
Light changes after 18 weeks have no affect
"In terms of the control of sexual maturity modern hybrids are virtually non-responsive to an increase in daylength by 18 weeks," Dr Lewis asserted. "It takes about two weeks for a pullet to develop a mature ovary and oviduct and about 3-4 weeks for a flock to reach fifty per cent production. And so, where sexual maturity is determined by an increase in daylength, age at fifty per cent lay will occur at least five weeks after the increase has been given." But, went on the consultant to the Shaver breed company, the latest survey of Shaver 579 performance showed that most flocks reached fifty per cent at 20-21 weeks. "It follows therefore," he concluded, "that the majority of these birds were probably maturing in response to the rearing daylength, usually 10 hours." But then, he said, a pullet actually needs no increase in daylength to trigger sexual maturation. "When it's big enough, old enough and exceeds a threshold of fat deposition it will lay."
Pale eggs are caused by heat not light
The universally held belief among free range producers that pale eggs and thin shells are caused by the bright light of the sun is wrong, said Dr Lewis. The cause is heat stress.
"The relationship between thin shells and high temperature has been known for more than seventy years," he said, "and experimental exposure to abnormally high temperatures for as short a period as six hours has resulted in an immediate reduction in shell thickness in the next egg laid."
The reason, he explained, is that the heat produces changes in blood chemistry which in turn results in an early demand for calcium from the skeleton and a reduced absorption of calcium in the gut. Eggs also spend longer in the oviduct and variations in this effect—caused by daily changes in heat stress levels—cause disruption of the organisation of egg formation leading to shell abnormalities.
If all this is true why do birds in hot climates not always produce heat-stressed eggs? The reason, said Dr Lewis, is that they are acclimatised to the higher temperatures. In Britain birds that range freely and have shade from trees or hedges will modify the effects of heat stress.
But after all his thought-provoking views Dr Lewis came up with another piece of advice for producers who are happy with the way their current lighting plan is working. "If what you are doing is working," he said, "don't change it for change's sake."
