Genetic Progress Theory and Budgerigar Breeding.Much is written about pairing practices in the budgerigar world and most breeders would have an idea of the direction they wish to take their stud. We use visual assessment of our birds and pedigrees to determine appropriate pairings and we do so with the standard in our mind as this is our breeding goal, but few would think of their breeding program in the terms that are outlined here. A breeding program is just a gathering of pairings if it is not put in context and is framed by an achievable breeding goal. Irrespective of what you are breeding, genetic progress can be defined by the following equation: ΔG = (A i σg) I where: ΔG – Rate of Genetic Gain A – Selection Accuracy i – Selection Intensity σg – Genetic Variance I – Generation Interval (length) This equation is often referred to as the "breeder's equation". In most commercial livestock breeding programs, the breeder is assisted by the provision of breeding values such as BreedPlan for beef cattle, Lambplan for sheep breeders and ABVs for Dairy Cattle. These breeding values are based on phenotypic (visual) measurements, their variance from their contemporaries (herd or flock mates of the same age) and the historical performance of relatives. In the world of budgerigar breeding, we do not have the luxury of breeding values and are therefore left with phenotypic (visual) assessment of our birds and knowledge of their ancestry and family history/performance via pedigree. Where tools such as breeding values are not available for the breeder, the equation for genetic gain changes slightly to: ΔG = (h2 i σp) I where: ΔG – Rate of Genetic Gain h2 – Heritability i – Selection Intensity σp – Phenotypic Variance I – Generation Interval (length) Heritability replaces selection accuracy and phenotypic (observed) variance replaces genetic variance. So what are these things? h2 – Heritability. There are a number of ways of describing heritability. The definition I like is “the proportion of the variance between individuals that can be attributed to genetics". If you google heritability you will find a range of ways of describing it. The trick is to find the one that you understand. They all describe the same thing – just in different ways. In this instance we are talking about the relationship between how the bird appears and how it breeds, i.e. how closely do the offspring resemble the parents. i – Selection Intensity. Is exactly what it says. It is about how intensive your selection is. Breeders who do not try and breed as many chicks as possible are sacrificing selection intensity. The more birds that are bred and the less selected to be the parents of the next generation, the greater the selection intensity. Breeders who run a superior cock over a number of hens are increasing selection intensity. Selection intensity is about maximising the number offspring of your best birds. It is also about your focus on your breeding goal. Using birds in your program that do not display, or are not bred from birds that displayed the characteristics you are selecting for, is an indication of lowered selection intensity. σp – Phenotypic Variance. What we are talking about here is the observed differences between birds. In situations where breeders have breeding values to work with, the |
differences between individuals can be assessed on a genetic basis. In the absence of breeding values we need to rely on our eye and our assessment of birds in relation to the similarity or otherwise to “the Standard”. Variance in a population is a vital component of breeding. You cannot select for a trait if the trait does not exist or appear in you stud. If you want to breed for directional feather but do not possess birds that display directional feather, then your task will be difficult if not impossible. If you want a characteristic or trait in you stud and you cannot see it in your birds, you will most likely need to acquire it from elsewhere.
I – Generation Interval (length). One of the key drivers of genetic progress is Generation Interval. If you think of Heritability, Selection Intensity and Phenotypic Variance of the cogs of the machine, then Generation Interval is the handle that turns the cogs. The more often you turn the handle, the more progress you make. If you get the top line components of the equation wrong and maximise generation interval, then you will be sent down the wrong track faster. But, if you get them right and manipulate generation interval in your favour, you will make good progress. So how do you minimise (shorten) generation interval? Minimising generation interval is all about using your youngest birds. You need to identify the best birds early and integrate them into your breeding program. As we breed these birds to show, we are faced with the predicament do I show the bird or do I breed with it? That becomes the choice of the breeder. They key is to understand the trade off you are making. If I show the bird and delay breeding with it, then I lengthen the generation interval. It becomes a choice! In essence all of these factors need to be balanced in a breeding program. Maximising any one of these elements will not deliver the desired result. To maximise accuracy of selection (heritability) we compromise generation interval. This is done because the accuracy of our selections increases with breeding knowledge. Does the bird breed “truly” (like itself) or does it breed better or worse. The only way to assess this is over time by evaluating the bird's offspring. So there is a trade off between accuracy of selection (heritability) and generation interval. Similarly, if we maximise selection intensity and only use the very best we have available (e.g. one cock over five hens) then we limit genetic and phenotypic variation. We start to restrict the pedigree (gene) base of the stud. We make the stud more related, and we potentially limit some of the characteristics that might be lying “inert” in the birds we have chosen not to breed with. A word on inbreeding. Inbreeding in most modern livestock breeding programs is avoided. This is because the architects of these programs have tools such as breeding values that allow you to better identify the genetics of the animals you are working with. Inbreeding allows you to “recreate characteristics within your stud” by combining the genes and gene sequences that lead to the appearance of the characteristics in the first place. By “doubling up” on birds in a pedigree that display the desired characteristics, you increase the chance of the characteristics re-appearing. Inbreeding is not a tool that allows you to progress. It only allows you to recreate. It is a great tool for cementing a characteristic within your stud. But it does not allow you to introduce the characteristics to your stud that you may be lacking. Budgie breeders rely on pedigrees, visual assessment and scrutiny of breeding performance when making pairing decisions. Inbreeding is a great tool for establishing a “line” however for that line to progress, new “blood” needs to be introduced. Inbreeding has a down side. Loss of vigour, fertility, disease resistance and size are all consequences of inbreeding. These phenomena are well documented in other species. Budgies are no different. Balance is the key. As budgerigar breeders, we need to find the balance between the components of the breeder's equation as well as the balance between inbreeding and out crossing. Good luck with your breeding! |