Genetic progress - what controls it? (part2)

Selection Differential (SD)

• This is the real power in the breeder’s pack as it’s where you can put emphasis on the trait to be improved.
• Think of selection as a pressure that you can increase or decrease.
• It’s defined as the superiority of the selected parents for the next generation over the population average from which they came.
• It’s a measure of how good the parents of the next generation will be, and you measure this against the flock or herd mean.
• Consider the males and females separately as you can be much more selective among the males as fewer are needed.
• Example:
  • Sheep flock average growth = 100g/day
  • Selected males average growth = 250g/day.  Their SD is 250-100 = 150g/day
  • Selected females average growth = 150g/day.  Their SC is150-100 = 50g/day
• The combined SD is 150 + 100 = 250/2 = 125g/day
• Now you can seen the advantage of Artificial Insemination (AI) where only one superior male is needed to mate the population, or of multiple ovulation and embryo transfer (MOET) from the top female.  These techniques can greatly increase selection pressure.
• The key point is to pick parents as far above the population mean as possible, because if you breed average males to average females – you will get average offspring and no progress will have been made.

Generation Interval (GI)

• This is the time interval between generations.   It’s defined as the average age of the parents when the offspring are born.
• To ensure a rapid turnover of generations, and faster improvement, this value should be short.
• However there are limits as breeding is not possible (without technology) before the parents are sexually mature.
• Waiting for performance records also lengthens the GI, eg waiting till a bull’s daughters have completed a first lactation, or till a dam has shown good lifetime performance.
• To keep GI short, you are torn between making early decisions on parents, and hoping that they indicate good lifetime performance.

Repeatability

• We talk about the “repeatability” of a trait – whether it is high or low.
• For example we know that the first fleece weight from a sheep at 14 months accurately indicates lifetime wool production.  And first lactation milk production in cattle is a good measure of lifetime yield.

Limit what you want

• Two things challenge you in breeding.  First deciding what you want to breed for and secondly trying to keep your wish list short.
• A basic truth of breeding is that the more traits you select for, the slower is the rate of improvement in any one of them.
• So the message is clear, if you want to see progress – keep the list of desired traits short. 
• Two or three is a good number to aim for – but one is ideal if you are brave enough.
• So when facing this decision, you can see how the formula about genetic gain/year is so important.