Understanding the Role of the Skeleton in Egg Production. Part 206 May 2014
The second part of this article from HyLine describes the importance of good quality pullets for achieving healthy growth and productivity, what happens to the skeleton at sexual maturity, bone quality during the productive phase and how to monitor keel bone integrity in hens.
The Importance of Good Quality Pullets
A strong skeleton starts with good pullet quality. Best management practices should always be used when growing pullets. Please refer to the Hy-Line International Technical Update “Growing Management of Commercial Pullets” for information regarding pullet programmes.
Chicks hatch with relatively underdeveloped internal organs and systems. The main systems that are developing in the initial weeks after hatch are the intestinal tract, the immune system and the integument (skin and feathers). The development of the intestine is crucial for nutrient absorption and will determine a hen’s future production efficiency. Strong intestinal development will also strengthen the immune system and minimise the possibility of future enteric diseases.
Starting at about six weeks of age, pullets have a more mature intestinal tract and immune system and also are able to regulate body temperature, which allows for more energy to be allocated for growth. The fastest rate of growth for the skeleton occurs between six and 12 weeks of age. During this time period, layer pullets gain an average of 90 to 110 grams of bodyweight per week. By 12 weeks of age, the skeleton is 95 per cent developed and once the bone growth plates close around the time of sexual maturity, no more bone length can be added. Any delay in growth will affect the size of the mature bird and delay the onset of production.
At 13 weeks of age, pullets have reached about 95 per cent of their adult size but only 75 per cent of their mature weight. Over the next six weeks, muscle, medullary bone and reproductive tract development will constitute much of the weight gain. Once birds have reached the appropriate level of development as determined by bodyweight, the flock will be ready for light stimulation to start egg production. Laying hens will continue to add muscle and bone mass and gain weight until around 32 weeks when the full mature bodyweight is achieved.
A pullet flock that experiences higher levels of stress during periods of rapid growth is more likely to have poor uniformity that can affect peak production. When a flock lacks uniformity, the lighter birds will not come into production for up to 10 weeks after birds that meet or exceed ideal bodyweights. Therefore, while most of the flock may be laying at 96 per cent, the five per cent of the flock that is underweight may be laying sporadically (or not at all) and will reduce the peak production percentage. Stressors that may be avoided include moving, injecting with inactivated vaccine, wide ranges of environmental conditions inside the house, poor feed quality, crowding and any other abrupt changes in routine.
Crowding in the pullet house usually starts to affect birds at approximately 10 to 12 weeks when the skeleton is near full size. Crowded pullets will have issues with uniformity and body weight gain until transfer. Refer to the Performance Standards Manual for rearing space recommendations.
Measuring body weight gain and uniformity is an excellent method for tracking flock growth throughout pullet growing. Higher bodyweights result in larger skeletons and more muscle mass which leads to better production. Waiting to light-stimulate until target weights are met is the most effective solution to ensure good persistency of lay and avoid a post-peak dip in production.
Monitoring bodyweights every week should not stop at transfer. It is ideal to weigh every week up to 32 weeks of age and at least every two to four weeks until the end of lay. This practice will give an indication if nutrient intake is sufficient to support production, growth and maintenance requirements of the bird.
Sexual Maturity in the Laying Hen
About two weeks prior to egg production, the hen will undergo sexual maturity. An increase of oestrogen will stimulate development of the oviduct, reddening of the comb and wattles and a complete transition from building skeletal bone to building medullary bone.
To aid the growth of medullary bone before the first egg, it is recommended to introduce more calcium through the use of a Pre-Lay Diet. Constantly improving genetics provide producers with layers capable of very high peaks of lay and good persistency. To ensure that genetic potential is reached, building medullary bone and formulating diets with sufficient nutrient density to meet the daily requirements of the bird is very important. However, there can be a negative impact on feed consumption from the sudden increase in dietary calcium levels of one per cent to above four per cent at the start of lay. Field experience indicates that the use of Pre-Lay Diets helps as a transition between the Developer and the Peaking Diet.
Correct feed formulation and matching diet density with consumption will minimise the impact of reduced calcification of bone over the laying cycle and extend the persistency of shell quality.
Bone Quality During Production
The length and width of the poultry skeleton is complete when the hen has started to lay. However, the bone mineral density and content, as well as the ratio of cortical, trabecular and medullary bones can change dramatically. The laying hen skeleton is strongly influenced by level of egg production, diet formulation in relation to consumption and disease status. A well-grown laying hen will typically not face skeletal issues until after peak, even with mildly to moderately deficient diets. With an underweight laying hen, nutrient deficiency will more quickly affect flock results. Mild to moderate nutrient deficiencies will usually cause skeletal and/or shell quality issues first with production issues following. Severe nutrient deficiency will still cause noticeable and rapid drops in production.
Many animals, including birds, experience thinning of the cortical and trabecular bone thickness with age. Laying hens also experience changes in overall bone strength. Although medullary bone is the most labile bone type, if a hen is calcium-deficient, cortical and trabecular bone will also be mobilised as a calcium source. During the laying period, hens have been shown to have a net increase in bone mass as a result of medullary bone formation and the loss of structural bone. However, without a moult or cessation of production due to nutrient deficiencies, the constant high level of oestrogen in a laying hen will prevent the repair of structural bone. Loss of cortical bone can result in crooked keel or bone fractures, both of which are detrimental to hen welfare and production.
As birds age, medullary bone content will increase while structural bone integrity typically decreases. Birds that lose too much cortical bone and gain medullary bone can have good eggshell quality but are at a higher risk for keel curvature or bone fracture. The best way to minimise the loss of cortical bone is to ensure that the correct levels of calcium, phosphorus and vitamin D3 are fed throughout the lay cycle.
Preventing loss of skeletal integrity starts with the pullet but continues through the life of the bird. It is also important to understand the clinical signs of osteomalacia, osteoporosis and osteopenia and make the appropriate changes as soon as possible.
Monitoring Skeletal Integrity
The best way to minimise the impact of soft bones in a flock is to monitor regularly. Handling birds while weighing is the ideal way to collect this information. Select birds from the same cage, colony or section of the barn at least every four weeks to ensure consistency of data and provide early notice if crooked keels are observed.
- Keels are scored by feel and observation. The Hy-Line method is to score on a four-point scale based on normal (#1), mild (#2), moderate (#3) or severe (#4) curvature.
- At the beginning of calcium, phosphorus or vitamin D3 deficiencies, keels may be flexible but not yet curved. This is an important clinical sign to note.
- Hens with recalcified, curved keels would indicate a nutritional deficiency earlier in the life of the flock.
- It is ideal to handle a minimum of 10 birds in at least two or three areas of a barn.
Overall, it is ideal to have greater than 90 per cent of birds handled in the Score 1 or Score 2 category. More than 10 per cent Score 3 or Score 4 birds, or increasing numbers every week, indicate the possibility of an issue.
The final part of this publication will be available on ThePoultrySite next week.
Go to our previous article this publication by clicking here.
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