At 12 weeks the frame size of broiler breeders is over 90% complete. From that moment the body condition or fleshing should gradually increase. To really know if the birds have the right fleshing, the birds have to be handled. The more birds that are handled at different ages, the better the condition of the birds will be understood. Over the last few years broiler genetics have advanced, particularly in feed conversion, daily weight gain and meat yield. On the other hand, for broiler breeders to produce a lot of hatching eggs weight gain needs to be controlled to optimize performance. In rearing, both body weight and uniformity are often monitored; these characteristics are vital to achieve good peak production and consistency. Another important tool used more and more is the monitoring of the development of fleshing. The birds have to be well prepared in rearing so they have, at the right age, the correct body conformation with enough fat reserves at the moment of light stimulation. The transition from pullet to a highly productive female involves a change in how energy is proportioned between skeletal growth, muscle deposition, fat deposition and development of reproductive organs for egg production. This article describes how to prepare the pullet at the right moment for light stimulation. Rearing has four important periods. Frame size and uniformity are determined for most of the flock’s life from day one to week eight. From there until week 12 is the time for a controlled feeding program. Puberty begins as of 12 weeks and the flock should start to slowly increase fleshing and overall condition. In the weeks 16-20 the flock needs to accelerate growth rate to prepare for sexual development and achieve proper uniformity and fleshing. Uniformity and Early Development The first priority is to get good carcass uniformity. When this is achieved in the first eight weeks of life, it is easier to bring all the birds in the right condition, with the correct amount of fleshing and be able to stimulate the whole flock with light stimulation. The first seven days after placement are critical to achieve good uniformity. Enough feeding space and easy access to water should be provided for the chicks with the correct ambient temperature. Then 48 hours after placement a minimum of 95% of the chicks should have their crops filled with water and feed. One method to obtain good uniformity is to place the birds at three weeks into three groups. Take one group of average birds, one group of small birds and one group of heavy birds. Feed the birds the necessary amount of feed so that all birds have the same bodyweight at eight weeks. Keep the birds separate in rearing and production if possible. The smaller birds are in general the slower eating birds and the heavier birds are the fast eaters. It has been noticed with mash feed that the smaller birds eat for 30 minutes longer on the same amount of feed than the heavier birds. The uniformity should increase during flock life – and never decrease. Uniformity should be a minimum of 70% (+/- 10). When this level of uniformity is not achieved, a second grading is recommended. To maintain good uniformity, the distribution of the feed is critical. It should be distributed within three minutes. A good practical solution is to provide the first feed with a pan feeder system or a chain system in the dark, or use the signal light feeding method to get good, fast feed distribution. Signal light feeding teaches the birds to relate feed distribution with a sign. The learning process starts on week three when feed restriction starts. The house lights are switched on about 30 minutes before feeding time and the farmer goes round to check if everything is OK. The lights are then switched off for one minute; and then the signal light bulb at the end of the house is switched on for one minute – this is the signal. The feeders start and after the feed is distributed the house lights are switched on. Development of Fleshing What do we mean with the term ‘fleshing’? This is an indication of how much muscle is developed on the breast and wings. Proper fleshing plays a role in the ability of a hen to lay a high number of eggs. The birds have to build up enough fleshing to deposit fat in the period from 18-21 weeks of age. Pullets have, in the rearing period, different fleshing conformation over time (Table 1). Until four weeks of age, we would like to see pullets with a more rounded shape of breast; this we call a Fleshing 4 (Table 1). In the maintenance period, the body conformation should change to a Fleshing 2. In this period the breast muscle should not be prominent. From 12 weeks onwards the fleshing on the birds should gradually increase so they have a fleshing of 2-3 at 16 weeks, and 3-4 at 20 weeks. Method of fleshing To make the right decision during the different periods of rearing, an accurate assessment of fleshing is important to validate what decisions you make as a breeder manager. How should this be done? A representative sample of the flock in the house should be collected. The fleshing evaluation should be done at a representative place in the house, for example in the middle of the house or at one end. When the birds are sampled close to where the feed enters the house, you normally catch the more well-fleshed birds. The fleshing should be done with a catchpen. The minimum number of birds is 50 per house. Each bird should be evaluated and given a fleshing score according to Table 1. After the fleshing is done, you should calculate the percentage of the birds in each fleshing group. A good practice is to do this together with bird weighing. How to achieve a good fleshing At 12 weeks the frame size of the bird is over 90% complete. From that moment the body condition or fleshing should gradually increase. Just to monitor the body weight is not enough. Over time more management items have been added to regular evaluation of the flock. From only using body weight, fleshing and later pelvic or vein fat, we now also use feather drop in the wings to see if bird development is on track. The reason is that the body weight curve has changed little over time, but genetic potential has been increased year on year. If there is excessive restriction, the feed intake will be limited, which could result in insufficient intake of amino acids, energy and vitamins and minerals with a negative effect on the development of the adult feathers and fleshing conditions and at the end of the rearing also a delay in fat deposition. The carcass size has a big influence on the body weight composition. Where there are two birds with the same body weight but different carcass size, the smaller bird will normally have more fleshing. To know if the birds have the right fleshing, the birds have to be handled. This should be done at several ages of the rearing flock, with the best ages 12, 16, 19 and 21 weeks. The more birds handled at different ages, the better the condition of the birds will be understood. The table is only a guide but indicates the importance of starting early (12 weeks) to evaluate the pullets and determine if the flock is on target from the beginning. The female’s body composition at lighting is just as important as the bird’s body weight, which means that the hen must have adequate fat reserve and fleshing at this point. Birds normally lay down fleshing quite easily between 16 and 20 weeks, however this is not the case with building fat reserve. It has been reported by different researchers that the abdominal fat content in broiler breeders has decreased during the past 20 years from 3.3 to 0.5%. Enough protein and energy intake in the first 10 to 12 weeks is important for the proper body conformation at light stimulation. Feed formulations have to be considered in relation to body weight and feeding curves. Nutritionists do not often get enough time to go into the field to check fleshing and feathering and general condition of the bird and to see how their feed is working out. The field managers need to become an important link between field situation and the feed mill or nutritionists. Try to avoid having a high percentage of birds in low condition with fleshing #2 at 12 weeks of age because these birds are impacted more by oil vaccinations around 12 weeks of age, and normally are still behind in development at 16 weeks of age. To build an adequate amount of fat deposition, the female must have sufficient weight gains in this critical 16 to 20 week period. A good management tool is to have a 33% to 38% increase in female bodyweight during this period. Birds that are then overweight should never be brought back to the standard weight at 20 weeks. If a pullet is 100 g overweight at 16 weeks, it should also be 100 g overweight at 20 weeks. Light stimulation Birds should be light stimulated only when they have enough condition and fat reserves. At the moment of light stimulation we like to see that the birds meet the criteria of an average BW of 2.4 kg, more than 95% of the birds should have a condition of #3 + #4 (majority in #3) and 90% or more of the birds should have fat on the pelvic bone or show some vein fat. If the females are not ready for light stimulation, this should be delayed for one week. Tables 2 and 3 are two different field examples, which were evaluated at 21 weeks. The flock in Table 2 was delayed for one week with light stimulation so the birds had the right condition and enough fat on the pelvic bones. The flock in Table 2 had reached peak production of 86.7% and Table 3 reached 88.2%.