Even among day-old chicks from the same breeder flock, there are some differences in initial body weight. As chicks grow, differences in individual health, physical condition and genetic traits cause their body weights to gradually diverge. When flock management alone cannot fully address a rapid decline in uniformity, it is necessary to divide the entire flock into groups to improve flock uniformity.
（1）Causes of poor uniformity
Common factors that cause uneven body weights include chick quality, feed quality, differences in feed particle size, humidity, immunization, the effects of formaldehyde gas when chicks are placed, mixing chicks from different sources at 1 days of age, poor-quality beak trimming, temperature (too high, too low or uneven), uneven feed distribution, incorrect feed quantities, excessive stocking density, insufficient water supply, feed energy levels that are too high or too low, insufficient lighting intensity during feeding, incorrect feed-line height, irregular feeding times, inaccurate flock counts or birds moving between pens, disease or bacterial infection, and other factors.
(2) Age at first grading
Broiler breeders are usually graded for the first time at 22–28 days of age. The 20–25% heaviest and lightest birds are selected and kept in separate pens, or the birds are divided into large, medium, and small groups. Feed quantities are adjusted according to the body weight of each group, with all birds brought to standard body weight over the next 4–5 weeks or reaching the body weight standard by 9–10 weeks of age. If breeder grading is carried out too late, there will not be enough time for the flock to regain uniformity and reach the body weight standard by 9–10 weeks of age.
At breeder farms with lower management standards, broiler breeders are graded for the first time at 7–10 days of age, and then graded a second time at 5–6 weeks of age.
(3) Pen Design
Before introducing the birds, the layout of the poultry houses and the pen arrangement inside them should take future grouping needs into account. Plan in advance how many groups are needed and where each group will be placed in the poultry house.
Before grouping, first weigh a sample representing 5－10％ of the birds in each house and record the actual weight of each sampled bird individually. Then determine the proportions of birds to be assigned to the small, medium, and large groups; for example, small birds 20％, medium birds 60％, and large birds 20％. From the actual weight records, identify the upper weight limit of the lightest 20％ of birds. This value will be the weight cutoff between the small and medium pens during grouping. Likewise, identify the lower weight limit of the heaviest 20％ of birds. This value will be the weight cutoff between the medium and large birds. Next, weigh every bird in the flock and group the large, medium, and small birds according to the cutoff values above.
When flocks from different rearing houses are transferred to the same laying house, the flocks from each house should have the same body weight at the time of transfer.
Keeping male and female birds in separate poultry houses makes feeding and management more convenient.
(4) Adjusting the body weight of the flock after sorting
After sorting, the large, medium, and small bird groups need to be reweighed and their average weights calculated. The actual weights should be marked on the weight curve. In accordance with the principle that all birds reach the manual weight standard by 9－10 weeks of age, draw a line from the average weight of each sorted group to the standard body weight at 9 or 10 weeks of age. This gives the new weight standard the group needs to gain toward over the next 4－5 weeks. For groups that are too heavy or too light after sorting, if it is difficult to catch up to the standard by 4－5 weeks, they must reach the manual weight standard no later than before 12 weeks. Because the reproductive system of breeder birds begins to develop after 12 weeks of age, and to ensure it develops fully after 12 weeks of age so that sexual maturity is achieved on schedule, the weekly weight gain of breeder birds after 12 weeks of age must be no less than the weekly weight gain level specified in the manual.
To ensure that the large, medium, and small birds reach the manual weight standard on time by 9－10 weeks of age, the weekly feed quantity for each group must be set after sorting according to its newly established weight standard. In general, the feed quantity for the medium-weight group can be increased normally from the previous amount; the increase for the light-weight group should be somewhat greater, while the increase for the heavy-weight group should be somewhat smaller. The aim is to ensure that each group gains weight in accordance with its newly established weight standard.
As a general rule, during 9－10 weeks of age after sorting, there is no need to select and adjust the larger and smaller birds within each weight group every week before the groups are adjusted to the manual weight standard. This helps ensure a strong correlation between weekly feed increases and weight gain.
If uniformity is still unsatisfactory after reaching the manual body-weight standard at 9－10 weeks of age, a small number of underweight birds in the flock can continue to be selected out for separate body-weight adjustment. To ensure that all birds achieve weekly weight gains at or above the manual standard after 12 weeks of age, the number of selected birds should not be too large. Otherwise, the average body weight of the large and medium-weight groups will be raised, misleading us into having to reduce their weekly weight gains and affecting the development and sexual maturity of the breeding stock. Therefore, avoid insufficient weekly weight gain in the breeding stock after 12 weeks of age, when the reproductive system is developing rapidly, as this can result in poor sexual development.
For flocks fed manually, we can determine the exact feed amounts for the large-, medium-, and small-weight groups based on the new body-weight standards established after sorting, and adjust the weekly feed increase as appropriate each week according to weight gain. For flocks fed mechanically, birds in each weight group can, where possible, be assigned to the same feed line. This can also effectively control and adjust the body weights of the different groups. Alternatively, when transferring the birds, the large-, medium-, and small-weight groups can be transferred to the same poultry house, making feeding and lighting management easier.
In actual production, we often feed flocks of two or more different body-weight types on the same feed line. This makes it very difficult to adjust body weight and determine the actual amount of feed consumed by the breeders within a limited time. In this situation, we can use the following three methods:
①Minimum feed-line quantity, with manual feed added. If the mechanical feeding equipment in use allows feed to be added manually, the amount of feed for automatic feeding can be calculated based on the minimum quantity for the entire feed line and the number of birds. For pens requiring more than the minimum quantity, the additional feed can be provided manually after automatic feeding on the line is complete. This method provides a relatively accurate amount of feed for each pen, and often achieves relatively good results in adjusting body weight.
② Adding a supplementary feed bin: When the mechanical feeding equipment in use cannot provide hand-fed feed, this can also be addressed by adding an extra bin for supplementary hand feeding. This method can also achieve good results, and the actual feed intake is relatively accurate.
③ Adjusting stocking density or feeder space: When using mechanical feeding equipment, another approach is to control the feed intake of flocks of different body weights by adjusting their stocking density or feeder space. Specifically, increase feeder space or reduce stocking density as appropriate for lighter-weight flocks to increase actual feed intake; reduce feeder space or increase stocking density as appropriate for heavier-weight flocks to lower actual feed intake. This helps adjust the body weights of flocks of different weights. The drawback of this method is that the exact feed intake of each pen of breeding chickens cannot be known. As a result, when experience is lacking, this method often produces less-than-ideal results in adjusting body weight or requires more time. The experience of technical management personnel is often the most important factor.