- ✓Take a weighing sample that truly represents the flock
- ✓Calculate the average daily gain and compare it with the benchmarks
- ✓Calculate and interpret the coefficient of variation (CV) of a flock
A sample that tells the truth
Lesson 1 of this track gives the basic rule: about 100 birds, in 3 places, on the same day and at the same time. The Cobb guide sets out the method so that the weighing does not lie:
- Divide the house into 3 sections (front, middle, back).
- In each section, weigh at random about 100 birds, or 1% of the flock.
- Set up a small catch pen and weigh every bird caught in the pen, without choosing, setting aside only the culls (runts, sick birds).
- Record each weight, one by one. The average alone does not let you calculate how even the flock is.
- Also weigh on the days when you plan to change the lighting programme: it is adjusted according to the birds’ average weight.
Source: Cobb.
Whoever catches “birds at random” unintentionally picks the calmest or the easiest to grab. By weighing the whole pen, you avoid this trap and the average matches the real flock.
Males and females do not grow alike
A flock of unsexed chicks mixes males and females. Males grow faster: a mixed flock is therefore never perfectly even, and that is normal.
| Age | Females | Males | Mixed flock |
|---|---|---|---|
| 21 days | 969 g | 1,046 g | 1,008 g |
| 28 days | 1,518 g | 1,690 g | 1,604 g |
| 35 days | 2,112 g | 2,425 g | 2,269 g |
| 42 days | 2,711 g | 3,186 g | 2,948 g |
Source: Efficiency Plus (performance objectives). Our broiler chicks follow the same benchmarks.
In practice: a sample taken only where the biggest birds gather (often males) gives an average that is too high. Hence the importance of the 3 sections and of weighing the whole pen.
Average daily gain
The average weight tells you where the flock stands; the daily gain tells you whether it is moving at the right pace. The calculation is simple: (this week’s average weight − last week’s average weight) ÷ 7.
| Age | Daily gain (benchmark) |
|---|---|
| 7 days | 32 g |
| 14 days | 56 g |
| 21 days | 77 g |
| 28 days | 90 g |
| 35 days | 97 g |
| 42 days | 97 g |
Source: Efficiency Plus (performance objectives), mixed flock.
According to the table, the flock goes from 1,008 g at 21 days to 1,604 g at 28 days: 1,604 − 1,008 = 596 g gained in 7 days, or about 85 g per day. If your flock gained only 50 g per day over the same week, it fell behind that week, even if its total weight still looks acceptable. Reminder: in an open-sided house in strong heat, 70 to 85% of the benchmarks is already a good result (lesson 1 of this track).
The coefficient of variation (CV)
Uniformity (the share of birds within ± 10% of the average weight, lesson 1) is completed by a second, more precise figure: the coefficient of variation, or CV. A low CV means an even flock; a high CV means an uneven flock.
- The standard deviation measures how widely the weights are spread around the average. In a normal flock, about 95% of birds fall between the average minus 2 standard deviations and the average plus 2 standard deviations.
- The CV is calculated as follows: CV (%) = standard deviation (g) ÷ average weight (g) × 100.
- No need to calculate it by hand: the standard deviation function of a spreadsheet (STDEV) or a calculator gives it from the list of weights.
- The CV lets you compare from one week to the next: you see straight away whether the flock becomes more uneven as it grows.
| CV | Corresponding uniformity (within ± 10%) | Assessment |
|---|---|---|
| 6% | about 90% | Very even |
| 8% | about 80% | Uniform flock |
| 10% | about 70% | Average |
| 12% | about 60% | Poor uniformity |
| 14% | about 52% | Very uneven |
Source: Cobb.
Average weight 1,600 g, standard deviation 128 g: CV = 128 ÷ 1,600 × 100 = 8%, i.e. about 80% uniformity: the flock is uniform. About 95% of the birds weigh between 1,600 − 256 = 1,344 g and 1,600 + 256 = 1,856 g. With a standard deviation of 192 g for the same average, the CV rises to 12%: poor uniformity, you need to find the cause.
When a figure slips: where to look
- Feeder space: if it is short, growth falls and uniformity is severely compromised. Cobb benchmark: 60 to 70 birds per 33 cm pan. Never let the feeders run empty.
- Cold litter at the start: it means less feed eaten early on, poorer growth and poor uniformity.
- Ammonia: it reduces weight and damages uniformity (lesson 8 of this track).
- Water: too few drinkers, set too high or too far away; a bird should never walk more than 3 m to drink.
- Light: a programme with a real night improves flock uniformity.
Source: Cobb.
For the entrepreneur
- ›Ask for three figures every week: average weight, daily gain since the last weighing and CV (or uniformity).
- ›A CV that rises from one week to the next is a warning, even when the average weight looks fine.
- ›Provide enough pans: 60 to 70 birds per 33 cm pan is the basis of an even flock.
For the technician
- ›Weigh in 3 sections, about 100 birds or 1% of the flock per section, weighing the whole catch pen.
- ›Record each weight, then calculate the average, the daily gain and the CV with the standard deviation function.
- ›If the CV goes above 10%, check feeder space, water, litter and ammonia.
The 3 questions to ask your technician this week
- How many birds did you weigh in each section, and did you weigh the whole pen?
- What is the average daily gain since the last weighing, and what does the benchmark say?
- What is the CV this week, and was it lower last week?
Key points
- •3 sections, about 100 birds or 1% of the flock per section; weigh the whole pen except the culls.
- •Males and females mixed: a flock is never perfectly even; at 35 days, 2,112 g for females and 2,425 g for males (benchmark).
- •Daily gain = (this week’s weight − last week’s weight) ÷ 7; about 85 g per day between 21 and 28 days according to the table.
- •CV (%) = standard deviation ÷ average weight × 100; CV 8% = uniform flock, 10% = average, 12% = poor.
- •About 95% of birds fall within ± 2 standard deviations of the average.
- •Too little feeder space, cold litter and ammonia damage uniformity.