Genetic Change, Part Four: Generational Interval

Part four of Genetic Change: Rules and Tools focuses on generational interval.

Genes do not get better or worse with age. Yet over time in a given population, the more consolidated the positive, dominant genes, or the more increased the frequency of genes under selection, the more rapidly change occurs.

Generation interval affects the rate of genetic change simply because the more rapidly one generation of improved alpacas replaces the previous one, the faster the gain.

 Mice reproduce more quickly than humans, producing 150 generations in the time it takes humans to produce one. (This is why mice are easier to improve than humans and are preferable laboratory animals.)

Generation interval is determined by the average age of 1) producing males and 2) females in a given herd. Alpacas have a generation interval of four to six years for females and approximately five years for males, although this interval will vary from herd to herd. The figure below identifies some common generation intervals for livestock.

The gains that result from lower genetic intervals are based on the assumption that when a breeder uses the scientific rules for gain, selection intensity, selection accuracy, and genetic variation they are producing improved versions or the previous generation, or more simply put the parents. It follows that the more rapidly you replace your breeding stock the faster the gain for the traits under selection.

EPD’s and generational interval

EPD’s allow a breeder, at the earliest possible point in time, to cull animals from their herd. They do not need to wait for years to determine the best breeding stock by trial and error. EPD’s rank their herd against the national herd every year.

This is particularly important when selecting replacement sires. Why breed a male for many years when it is only logical, and scientific, that his young sons, if mated to select females, will in fact be better studs than their father. Think of it this way: if you breed the same males to the same females every year the quality of your offspring will never improve.

EPD’s allow you to scientifically choose the best progeny from your production and then use them to create improved generations of alpacas. And remember this gain compounds, year after year.

The next series of alpaca articles that advocate for scientific selection and breeding systems for alpacas will explore the ABC’s of EPDs.

Part two of Genetic Change: Rules and Tools focuses on selection intensity. Stay tuned for the next two parts of this series.

Don Julio Barreda culled 10% of his entire herd every year, year in and year out. Culling is the first step toward intense selection within a population.

Selection intensity means being highly selective of the progeny produced by the high-quality parents you have chosen for foundation stock, and retaining in your herd only the offspring that exhibit a superior expression of the heritable qualities for which you are selecting. This ensures that breeding values will remain high and that each generation of offspring should improve.

When deciding which offspring to keep, breeders may choose to select animals scoring in the top 50% on of their EPD’s for a given trait or combination of traits. Later, they may select more intensely, selecting only those alpacas scoring in the top 20% of the EPDs.

The greater the selection intensity, the more rapid the rate of genetic gain. Remember, genetic change or the rate of gain compounds.

Here are the steps an alpaca breeder, at a minimum, should go through. Inspect the alpaca under selection in the following list (no particular order) for the following traits:

• Conformation
  • Bite
  • Legs (including movement)
  • Topline
  • Tail
  • Size
  • Testicles
  • Vulva
  • Toes
  • Ears
  • Hernia
  • Teats
• Color uniformity (commercial traits)
  • Pinto
  • Color contamination
  • Skin pigment
  • Variegation of the same color
• Birthing capacity and success
• Breed type or look
• EPD’s for each trait under selection

Alpacas not scoring well on the first four categories above should be culled from the breeding program, period. 

From the remaining animals under selection a breeders needs to determine, based on EPD performance, how many they will retain.

In my opinion, under no circumstances should a breeder use a male or stud to breed who does not score better than 90% of the males (by color) in the USA. I would never use any that were not in the top 2% for the traits I was emphasizing most strongly.

In Part 3 we will discuss how genetic variation impacts genetic gain.

Part One of Genetic Change: Rules and Tools focuses on selection accuracy. Stay tuned for the next three parts of this series.

Selection accuracy is important if any improvement or gain is to be made. This means the traits you select for must be heritable, and the animals you choose for parents must have high breeding value for the traits under selection. For instance, if you select for a heritable characteristic, such as fleece weight, you must use stud males who historically have produced offspring with higher than average fleece weights. The same goes for fineness, crimp, staple length, etc.

The following chart is the most recent from the AOA EPD program. As you can see, the commercially important traits are highly heritable. Any trait that is in excess of .40 percent heritable is considered to be moderately heritable verging on highly heritable. Although fleece weight is under .40 it is generally considered moderately to highly heritable in most fiber animals. I believe that lower number is due to variability in shearing methods and how people report the weight; some people do not shear and report the entire fleece.

If you purchase foundation stock, replacements, or breeding services, and fleece weight is your goal, you must ask for records of shear weights for all the male’s offspring and inspect as many of those offspring in a given environment as possible. Or better yet, before you buy breeding stock, you might want to ask if the breeder has EPD’s for the trait under selection.

EPD’s and Selection Accuracy

By inspecting the EPD, you determine the breeding value of the parents. Parents with high EPD’s for a trait assure a higher degree of selection accuracy for the progeny.

There are four ways or tools to help make accurate selection decisions:

1. Phenotype: Many breeders believe they can look at an alpaca – its fleece style, size, show winnings, etc. – and assess the breeding value of the animal under selection. Phenotype is the least reliable way to make accurate breeding decisions. Selection by phenotype alone results in accurate decisions less than 50% of the time.
2. Pedigree: Many breeders study pedigrees and swear that they can use them to make accurate selection decisions, and they do help, especially if there is line breeding or close relationships in the pedigree. Pedigree, when combined with phenotypic evaluation, results in accurate decision about 60% of the time.
3. Assessing Progeny: Most breeders do not assess a representative number of a parents offspring, but it is scientifically established that if they were to analyze just eight progeny together with their phenotypic analysis and a review of the pedigrees, their accuracy would increase to about 85%.
4. EPD’s: It is a scientific fact that adding EPD’s to your selection decision increases the accuracy of your selections to over 98%.

You can argue these points all you may, but the science is settled. Ignore it at your own peril. Ask yourself which percentage of accuracy would you like to be reflected in your breeding decisions: 50%, 60%, 85% or 98%?