Challenges of the increased laying persistency of the modern laying hen
(As published in The Poultry Bulletin – September 2016) Genetic companies have been very successful in selecting traits that increase the laying persistency of hens. Their motivation for this has been our need to be able to produce more food in a sustainable way. It is possible to reduce the size of the international breeding flock if the hens can produce more high quality eggs over an extended period while at the same time remaining healthy. Fewer hens will result in environmental savings through reduced nitrogen production as well as a reduction in the amount of food required to maintain these hens. Egg shell quality has a direct influence on the number of marketable eggs and this and, to a lesser extent, the egg weight influences the economics of egg production. Internal egg quality is important to the consumer and these parameters often deteriorate as the hens get older. This also brings about health and welfare challenges such as osteoporosis and reproductive diseases. Careful bird management and the correct nutrition that focuses on achieving the correct body weight and body composition at sexual maturity, and on supplying the correct balance of nutrients throughout the life of the hen is what is required to be able to meet these challenges.
How the pullet is fed during the rearing period influences its growth curve and hence its live weight and body composition at point of lay. The target is to achieve the adult weight before egg production begins. Birds which have a low body weight do not have the reserves and stamina to successfully function as a laying hen. A portion of the nutrients will then be used to complete growth rather than be allocated to egg formation, reducing the total number of eggs she will produce. There is a correlation between the initial weight of the hen and the total egg output during the entire period. There is also a correlation between egg size and the initial weight with an increase of 0,7 g for each 100 g gain in live weight.
The shape of the pullet’s growth curve is important. During the first 11 weeks the emphasis is on muscle growth, intermuscular fat deposition and skeletal growth. To encourage feed intake and the growth of the pullet during this period it is advised that crumbled or pelleted feed should be fed for the first six weeks. A good quality mash can then be fed from then onwards. From 11 weeks of age through to the onset of sexual maturity, protein deposition is mainly allocated to the development of the reproductive tract and fat is largely deposited in the abdominal fat pad. From 16 weeks through to the start of laying, the feed must encourage feed intake and this can be done by feeding of coarse particles, whole cereals and/or insoluble fibre which helps with the development of the upper digestive tract. The feed must be appetising and always available as the medullary bone reserves are being formed and the ovaries and oviduct are developing.
The high productivity of the modern hen has been achieved largely by selecting for an increase in the persistence of egg laying. Correct feeding limits the increase in egg weight with age and improves egg shell quality. The first challenge during the first stage of lay is to adjust the energy and protein requirements to optimise egg output and to control body weight. The requirement for growth is only present for the first few weeks at the onset of egg production. Thereafter, energy required for maintenance will depend on body weight and feather coverage and will increase with age. This allows for higher energy diets to be fed during this period which encourages the development of heavier eggs without depositing excess fat. Once egg production has become established and growth has stopped the energy requirement decreases and a lower energy diet can be used to minimise fat deposition.
Even though egg weight may not seem to be a major factor in affecting egg quality it is in fact very important. Generally, egg weight will increase as the hen gets older and this has consequences on trying to extend her productive life. As the egg weight increases shell thickness decreases and breaking strength is lowered, which effects the number of marketable eggs. There also seems to be a correlation between the colour of the shell and an increase in egg size with larger eggs being lighter in colour. Egg weight can also be increased by feeding diets which have a higher protein to energy ratio. It is important to adjust this ratio relative to the actual egg weight to optimise production. Failure to do so can result in egg weight increasing significantly and laying persistency will not be achieved.
Hens have a preference for larger particles. This is a problem when there is a large variation in the particle size but it is not the case when the feed is more homogeneous. Irrespective of the uniformity of the particle size the smaller particles disappear from the trough more slowly. These smaller particles usually contain the micro nutrients such as vitamins, minerals and amino acids which settle at the bottom of the feeding trough. The addition of small quantities of fat can reduce the problem by sticking the particles together. It is recommended that the feeder lines are stopped for a period each day to force the hens to ‘clean up’ this fine material. If this is not done, it can have an effect on the nutrition of the hen and the long term persistency of lay.
Laying hens often suffer from bone fractures (legs, wings) or deformities (keel) caused by osteoporosis, making this one of the major welfare challenges in the egg production industry. Hens have two forms of bone; structural bone is the strength of the bone and medullary bone which a storage area of bone and is drawn on for shell formation when it is required. Osteoporosis or loss of structural bone brings about weakening of the skeleton over the laying period and is a major contributor to the high incidences of bone fractures that occur in laying hens. The issue of bone strength and bone breakage and egg quality are intimately linked and nutrition has an important role in ensuring that balance. Osteoporosis occurs when the hen starts to draw minerals from the structural bone which are not easily replaced. This is caused by not providing sufficient dietary calcium during the rearing and laying period and it has an adverse effect on shell quality. On average a laying hen requires 2,2 g of dietary calcium to produce an egg. About two-thirds are supplied via the diet and the remaining third is drawn from her medullary bones.
The issue of bone strength and bone breakage and egg quality are intimately linked and nutrition has an important role in ensuring that balance.
After ovulation, egg formation begins with the formation of the yolk and albumen, which requires protein and energy deposition to occur early in the day. This requirement coincides with the peak feed consumption period. Shell formation begins in the late afternoon through to the early hours of the morning. During this time, feed intake is minimal and meeting the calcium requirement depends on calcium absorbed from feed earlier and on calcium released from the medullary bone reserves.
Management practices require hens to have about 16 hours of light per day. Light is not provided during the night and consequently there is little or no feed intake during this time, limiting the amount of dietary calcium available for shell formation. To overcome this, coarse limestone particles should be added to the diet. These particles are digested slowly and provide a source of dietary calcium during this period of peak demand. The calcium content of the diet should be calculated based on egg weight and the feed intake of the hens to optimise shell quality.
Reproductive tract and related diseases
With the improvement in the persistency the birds must remain healthy throughout the production period. A reason to cull a flock is usually due to a decline in egg numbers combined with a deterioration in shell quality. The long-term maintenance of the tissues and organs involved in producing eggs is required to extend the laying cycle of commercial flocks. Poor bird health and environmental stress will have an effect on egg formation and on the ability of the hen to maintain persistency.
References are available from the author on request.