Broiler breeders have the ability to consume large quantities of feed, as in essence they have the characteristics of broilers. Usually we tend to not restrict them too much in the first weeks of rearing, to achieve a good body weight and large frame size. To avoid the birds from becoming too heavy, it is often necessary to restrict the birds more severe after those first initial weeks. And often in the last weeks of rearing we then need to increase the feed again, to avoid that the birds will remain not enough developed and start production too late.
This often results in an S-shape of the feeding profile: in the first weeks the feed level and the increases are quite severe, than from 5-6 weeks to 14-15 weeks hardly any increases are given, and from 15-16 weeks onwards the increases are enlarged again. This S-shape, which even can be found in the feed profiles recommended in the breeder management guides, is the result of that increased feed level at the start. Feeding higher levels than the birds actually need results in more restriction in the period afterwards, as the birds are for a period of time overfed and needs to be corrected. One would expect that this will reflect immediately in extra body weight but that is actually not the case, as it takes time before body weights respond to feed amounts.
Although the general assumption is that feed amounts are less important than achieving the correct body weight, the consequences of this S-shape feed curve are quite substantial, even if the correct body weights are achieved. When we calculate the energy needed for maintenance, it shows that in rearing every 100 gram of more body weights require approximately 3 grams more feed. As the birds are growing roughly 100 grams per week, it means that they need every week 3 grams more feed. If we give less than those 3 grams, the birds will after a couple of weeks become more and more “hungry”. As they are restricted they will always feel the desire to eat more, but the relative feeling of hunger will increase if we do not give the 3 gram increase that is required for the increased maintenance. One would expect that the birds will then immediately stop growing, but that is not the case as their feed level is still higher than needed, due to the high feed amounts at the start. But after several weeks of no or small feed increases, they will get more and more nervous as they are falling more and more behind on what they need, and at a certain moment their feed level will be so far compensated that they start to be underfed. That doesn’t immediately result in underweight, as also then it takes several weeks before the birds really start to react with body weight. Once we see that the bodyweights do not increase enough, usually around 13-14 weeks, we increase the feed and if we are not careful end up in overfeeding again.
But a S-curve feed profile means that the birds go from relative overfeeding to underfeeding and back to overfeeding, an effect that I call “frog feeding”. And the negative effects of this “frog feeding” are quite substantial. First of all will it result in nervous birds, as they experience stress due to that continuous feeling of increasing hunger. They will have the tendency to start pecking at feathers and at each other, as they are looking for something to satisfy their needs. Not only that, but it also interferes with the development during puberty, the period of sexual development. Puberty in birds is roughly from 11 to 20 weeks, and if in the first weeks of that puberty they are seriously underfed, they will not have a smooth sexual development. In the last period of their puberty they will then be overfed to compensate, but as development takes time it will not fully compensate. As a result the birds might come in production later than expected, although they have the required body weights at transfer.
When we want to “cater” for the bird’s needs, we have to respect the fact that 100 grams of extra body weight requires 3 grams more feed. That means that during the whole period of rearing, a minimum weekly feed increase of 3 grams should be given. That means that we have to limit the feed amount at the start as quickly as possible, because otherwise we can’t give the feed increases required. One would expect that this will result in a complete different body weight curve, but that is actually not the case. The curves will alter slightly, a bit underweight at 5-6 weeks and on or slightly over the norm at 13-14 weeks of age, but not more than that. But the effects on behavior and development are quite substantial.
Once the birds start hormonal development (11 weeks of age) we need to go up in feed a little bit (4 gram weekly increase instead of 3 gram). At 16 weeks we have to go up again to 5 or 6 grams as the birds start to develop their sexual organs. The actual feed levels depend on feed quality, temperature, feed distribution etc, but the principle of steady increases resulting in a more linear feed profile remains.
This principle holds for all broiler breeds, but there is a difference. A female breeder like Ross 308 is early in life less eager to eat large quantities, which means that it is more or less automatically already doing a sort of linear feed curve with steady increases instead of a S-curve with very limited feed increases in the middle period. A bird like Cobb 500 is much more aggressive on the feed early in life and is therefore more likely to overeat in the beginning, resulting in very small increases in the middle period.
But the principle is equal for all breeds, and for both genders: during rearing a steady increase of minimum 3 grams per week will result in much more relaxed birds that are not or less nervous, pecking or flying, and have a better development during their puberty.