Genetics

Background 3158 views 1 commentupdate:Mar 9, 2016

Fit and well-proportioned broiler breeders

In breeders, achieving the recommended weekly body weight gain is the basis for good laying performance. But it is the right body composition that makes the difference between a high-performing breeder hen and a poor layer.

By virtue of its genetic make-up, a broiler has a voracious appetite to consume large amounts of feed and an equally matching capacity to convert this feed and thus gain weight. As the science of genetics progressively brings in each successive generation of broilers with increasing body weight gain and lower feed conversion ratios, it becomes increasingly difficult, but critical to maintain the body weights of the breeder stock within the levels specified for the breed. One will have learnt from experience that any inordinate deviation, either on the low or high side, from the body weights recommended during the growing and laying cycles leads to disastrous results. One will also have experienced that the most critical and most difficult part of managing a broiler breeder flock consists in achieving and maintaining the ideal body weight with the right proportion of skeleton, muscles and fat for any given age, from the time one starts with the chicks to the culling stage, as well as the right body composition.

Excessive body weight and body fat

It is a physiological fact that fat cells in the body are generated during the growing stages. Fat cells are like fuel tanks in which the body conserves any surplus energy that has not been utilised for future use. If the chicken ever experiences a feed restriction or starvation, these fat cells reduce in size but not in number. The moment an opportunity arises in the form of excess feed or a decline in production during the laying cycle, these fat cells soon expand. Even at times of starvation, fat levels in the body are not likely to completely disappear, because some level of healthy fat deposition is actually intended to protect tissues and organs. It is only the excess fat that poses problems like lowered production, poor fertility and increased susceptibility to environmental heat in chickens. This is also the reason that birds with extremely high body weights and associated excessive fat accumulation do not respond very well to forced moulting. You will also notice that when birds are being forced to moult, they first lose weight primarily through a loss of musculature, while very little loss of fat tissues takes place immediately. As we saw earlier, the number of fat cells in both fat and lean birds is more or less determined during the growing period. Once fat cells are present in the body, the increase in the volume of these cells is what causes body weight to rise in the adult laying bird. Whenever birds build up a large reserve of fat cells, there is an additional demand for energy which is used to feed the fat throughout the adult life. This is the rationale behind the guideline that, during the growing cycle, body weights should never be allowed to unduly cross the standards specified for any given age.

Photo: Henk Riswick
Breeder companies recommend just one, yet thorough grading around the 28th day in order to correct the inequalities that may show up during the first few weeks and thus gradually lead the flock to a healthy uniformity.

Maintenance, growth and reproduction

A significant portion of the total energy requirement is for the maintenance of all the vital body systems and functions, which include the physiological activities within the bird. This is critical as long as the chicken is alive, irrespective of its growth or reproductive status. Second to that is growth. And, finally, if the chicken has satisfactorily completed the stage of growth with optimum development and has sustained resources at its disposal for maintenance, then she is able to comfortably enter the next phase of the cycle, that is to say the reproduction phase.

If at any point the level of maintenance drops lower than what is required, the immediate casualties will be the next two phases, namely growth and reproduction. This is what actually happens in the case of birds that mature late and have body weights far below the standard specified for their age, as well as in the case of ‘drop outs’ or birds whose body weights fade for some reason. They somehow were unable to build up the required reserves in the form of right body weight for the age. Accurately recording and evaluating weekly body weights is therefore an indispensable routine that has to be given the highest priority in the management of broiler breeders, especially during the growing stage. On the other hand, birds which have grown much heavier than the set standard generally possess high amounts of fat tissue due to either faulty nutrition or poor feeding practices and therefore possess both a higher craving and need for additional feed.

Body weight and craving for feed

This phenomenon is far more complicated than it seems because it is linked to an intricate mechanism that is governed by the endocrine system. There are two major hormones that control the hunger and satisfaction centres in the brain and that are responsible for the ‘EAT’ and ‘STOP’ behaviour, namely Ghrelin and Leptin. Ghrelin is produced by the stomach and travels to the brain, where it ‘turns on’ the hunger centre, signalling the bird to ‘EAT’, while ‘turning off’ the satiety cell, which is meant to say ‘STOP’. Leptin, on the other hand, is produced by the fat cells of the body and travels to the brain, where it serves to ‘turn on’ the satiety centre while ‘turning off’ the hunger cell. Yet unwanted, high levels of Leptin coming from excessive body fat over a period of time make the satiety or the satisfaction centre insensitive or indifferent to this hormone. A a result, those birds that are obese continue to suffer from excessive hunger which, in turn, prompts a high internal demand for maintenance and so they tend to eat more whenever the opportunity arises. Such birds have unfortunately been pushed into a permanent vicious circle. So when we try to irrationally cut the feed allowance to them, we do so at the cost of physiological development and reproduction. This factor is even more critical when birds have started their production and some growth still remains to be completed, because such birds have simultaneous and multiple demands to meet all of the three functions, i.e. Maintenance, Growth and Reproduction. In the case of birds which have completed their growth, say after 30 weeks of age, the level of maintenance should be constantly sustained and monitored in order to make sure that the nutrition and health of the birds is always kept at optimum levels. This is where the representative sampling of body weight is of immense value, at least during the first phase of production, that is from 5% of lay to peak production, lasting approximately 10 to 12 weeks. Any abnormal drop in body weight at any time should be viewed very seriously as signalling a drop in the maintenance level and corrective measures should be taken. Similarly, an abnormal increase in body weight at this stage is an indication that the bird is getting more energy than it is spending.

Body weights, egg production and egg weights

Egg size is largely controlled by the size of the yolk that receives its material from

the liver. The size of the liver, in turn, depends on how well the skeletal framework, together with some healthy musculature, has harmoniously developed over the weeks of growth. The body weight and body condition of the bird as she comes into lay are the ultimate factors that will determine how good the performance of the breeder is going to be. Whenever body weights in pullets, as they advance in age, fall behind the standard for whatever reason, the general tendency of many managers is to ‘push in’ extra feed to boost weight gain, especially during the final weeks before the onset of lay. The result is that there is no healthy weight gain because such birds either will have not attained the right skeletal development for their age or, in most cases, may have already passed the stage of skeletal development before corrective steps could be taken. All of us know that, as birds advance in age, it is easier to alter the body weight than the skeletal size. In effect, with what I call a good amount of ‘tinkering’, pullets may grow with seemingly correct weights for their age after being forced into it, but because they have a small skeletal frame and a cramped body structure as a result, they will begin converting the excess dietary energy into fat. After all, where else can muscles develop except on the skeleton? Many such birds possess a small body structure with poorly developed internal organs and will not turn out to be consistent layers, since they do not possess sufficient, built-up reserves in the form of well-developed organs and muscles to fall back on. In short, they do not have what we would like to see, i.e. the right body composition. Birds that turn out to be poor layers very often come from such situations.

Photo: World Poultry
For good-quality fertilised hatching eggs, it is essential that the breeders' needs for maintenance and growth are met.

Correcting body weights

The ideal way to correct erratic body weights in Broiler Breeders is by GRADING the flock into different groups according to their body weights. This is a practical approach to restore uniformity over a certain period. As a healthy practice, most Breeder companies recommend just one, yet thorough grading around the 28th day in order to correct the inequalities that may show up during the first few weeks and thus gradually lead the flock to a healthy uniformity. Unfortunately, on many farms flock Grading is misused by some flock managers as a license to mess up the body weights through poor management, with the hope of correcting them later at frequent intervals. If, on the other hand, the farm manager scrupulously understands grading as a valuable tool for correcting management errors, he could be more proactive and concentrate on the basics of management to avoid these needless lapses, which involve additional costs in the form of feed and time, as well as irreparable damage to the physiology of the birds. One very important factor which has to be borne in mind while working out the revised plan for birds that have deviated from standards is to take into account the erratic pattern of weekly gain that birds have thus far experienced in order to make a smooth, but timely transition to the newly worked out standard curve. Very often, the body weights fall well below standard during the growing period. Many managers then resort to ‘tinkering’ in order to push up the weights to meet the standard within the shortest time possible, following no proper plan or schedule.
Do not forget that such birds, by virtue of being underweight in the early stages of growth, have ended up with a system that has the capacity of gaining only around 70 to 80 grams a week under a standard of around 110 to 120 grams. As we saw earlier, the result of overfeeding small birds is that they may succeed in attaining the standard weights at a given age but, given the poor skeletal frames for their age, they will be able to deposit only a limited amount of flesh on this stunted frame and will therefore gain weight more by way of fat rather than muscles. Breeder experts and knowledgeable flock managers will tell you that the right approach to take these small birds back to the standard is to plot out a revised plan on a graph whereby the weight gains are gradually raised every week above the actuals, so that over a period of time birds can go through a gradual and smooth recovery and catch up to the standard curve. This type of smooth correction allows for a recovery of both the skeletal frame size, to whatever extent is possible, and healthy fleshing.

What to do with heavy birds

With respect to the birds which have gained excessive body weights, it is necessary to scale down the rate of weekly body weight gains in a rational manner during a targeted period in such a way that the weights meet the standard curve at a fixed point in time, well before the growing stage is completed. This again requires the manager to plot a fresh graph on which the revised body weight profile is redrawn. Because the different abnormal situations involving body weight profiles that can arise within a growing flock are, along with the right techniques to correct them, very well-demonstrated in most breeder management manuals, it is needless to repeat them here. But one important principle to be remembered while correcting body weights is that the greater the deviation of the body weights from the standard, the longer time birds should be given to make a smooth return to the normal standard curve of body weights. Shortcuts will merely achieve superficial or temporary results and will only serve to satisfy records. What is important is the extent to which all the major systems that make up a healthy breeder have been gradually and harmoniously developing during the breeder’s growing period.

Register or log-in to view more articles like this from World Poultry issue 9 .

Leo Anthony

One comment

  • S. Lepelaars

    Very interesting, especially the part of body weight and craving for feed

Or register to be able to comment.