Applying Selection Pressure

This is the final installment on our two-part series on Stud Cattle Breeding essentials which covers, the Breed Standard, Setting Breeding Objectives, and Applying Selection Pressure.  With great thanks to Dr Ian Holmes BCVSC for his principal contribution to this article.

Applying Selection Pressure

Selection pressure in cattle breeding is a critical concept that shapes the future of your livestock production. This process involves the breeding of cattle to enhance desirable traits that meet the breed standard (in the case of stud breeders) and your  breeding objective (which is defined by your target market, environment and profit drivers). By applying selection pressure, breeders can direct the genetic makeup of their herds, ensuring that the best traits are passed on to future generations.

Applying selection pressure is about moving your herd towards your breeding objective and ultimately about ensuring market acceptance of progeny.  In essence the process relies on selection of superior parents for mating to produce offspring suited to your target market.

Any individual trait or criteria used in making your breeding decisions must:

• be of economic importance,
• be reasonably heritable,
• be able to be measured; and
• there must be variation in the trait so that superior animals can be identified and poor performing animals culled.

Economic importance can mean different things to different producers. For the commercial beef producer, typically the most important traits from an economic perspective will be fertility, growth rate and carcase quality. For the producer of stud cattle, there may be other additional traits that are important to economic returns.

Heritability is the degree to which an animal will transmit to its offspring the performance it displays in any trait. Some characteristics are controlled by single pairs of genes but many important characteristics are affected by multiple genes and so only slow progress towards the ideal can be achieved, eg food conversion efficiency.

You can’t easily move your herd towards the ideal if you cannot measure the traits. Some traits are easy to measure, and some are very difficult.  Measurement of traits can be assessed by observing the phenotype (how the animal looks) and the genotype (assessing the genes that the animal carries which will determine how the animal could potentially look or perform).

It is important to identify how much of the measured variation between animals is due to genetic differences between the animals and how much is due to non- genetic factors such as nutrition, herd management, disease control measures and all the other environmental factors which affect how the animal performs.  The higher the percentage of genetic variation, the greater the chance that traits will be passed on to the progeny.

The most important economic determinant in a commercial beef herd is fertility. Fertility is multifactorial and many of the factors are hard to measure and others are due to herd management, the environment or just luck. Therefore, putting selection pressure on every possible factor is neither practical nor productive.

Three fertility outcomes can be measured simply, and so putting ruthless selection pressure on these three traits will, over the generations, lead to a highly productive herd that is well suited to supply genetic seedstock for commercial herds. Those traits are calving ease, calf birth weight and pregnancy. If you cull all empty cows and those that deliver big calves (relative to the cow’s weight) or need assistance calving due to foetal oversize, then you will very soon have a very productive herd. This genetic progress will occur despite you not knowing or even needing to know, what has caused the cows to be empty or have calving problems.

Another increasingly important commercial factor is whether cattle are horned or polled. You can measure a horn phenotype easily and accurately as all horned cattle have two copies of the horn gene, HH.  So, if you want horned cattle, you can put 100% selection pressure on animals by selecting as your breeding stock only cattle that have horns. However, it gets more complicated if you want to select for polledness as some polled cattle will be “heterozygous”, HPc, i.e. carry both a polled gene, Pc, and a horned gene, H. Therefore, you cannot put 100% selection pressure for polledness just by looking at the phenotype of the animal. The actual selection pressure that you can put on depends on the proportion of PcPc and HPc animals in your breeding population.

DNA testing solves this problem. As polledness is determined by a single gene, if you DNA test just the sires used, you will gradually progress towards a genetically pure herd for polledness. But if you DNA test the cows as well as the bulls and cull all HPc animals, you can put 100% selection pressure on the herd and get a 100% polled herd in one generation.

In recent years, advancements in genetic testing and data analysis have revolutionised the way breeders approach selection pressure. Genetic testing is testing for a few specific genes, but “genomic testing” is where hundreds of thousands of genes are tested and attempts are made to link them to desirable and undesirable traits.

For most characteristics the genes only contribute ~10% to the final outcome and the vast array of environmental factors determine the other 90% of the breeding outcome (phenotype). Good managers will minimise these environmental factors which will enable the genes to express themselves more truly. So, you can’t put much selection pressure on body size or growth rate if you don’t manage your environment; you can have the best growth genes in the industry but they will not be expressed if you starve your stock.

Another complication is that some desirable genes oppose other desirable ones. Consider growth rate and marbling. Marbling and juiciness which is associated with good eating quality are due to fat. Fat takes nine times as much energy to produce as meat, after all it is a storage form of energy. Therefore, selecting only for growth rate means that you are putting selection pressure towards low fat meat which is generally dry and tough.

Consider also, selecting for growth rate results in larger calves which can lead to more calving difficulties. So, it is not sensible to put too much selection pressure on some individual traits. You need a balanced outcome which takes into account all of your breeding objectives. This is why selection indexes have been developed. There will be a range of these; a breeder must decide how important each trait is to them and so can create their own index by “weighting” the traits in their index accordingly. Or choose a breed specific published index that is most closely aligned with the breeding objectives of your herd.

Breedplan is a genetic evaluation system developed by the Animal Genetics & Breeding Unit which is a joint venture between the University of New England and NSW Department of Primary Industries. Breedplan estimates the breeding value of cattle and is used by beef cattle breeders to improve the genetics of their herds. It works by:

• using Best Linear Unbiased Prediction (BLUP) technology
• calculating Estimated Breeding Values (EBVs) for a range of traits, including weight, fertility, and carcase
• expressing EBVs in everyday units, such as kilograms (kg) of weight
• On average, half of the difference between an animal’s genetics and the genetic base is passed on to its progeny by the sire and the other half by the dam

Unlike the horn/poll gene, most characteristics are affected by the environment as well as genes. Body size (and growth rate) is determined by a few genes but is also heavily impacted by the “environment”. Examples of environmental factors that affect the mature size and growth rate of an individual animal are birth weight, calving difficulty, twin or single, born early or late in the calving season, milk production of the dam, scours or other diseases when young, stocking rate, pasture management, good or bad season, parasite burdens, teeth problems, lameness, water quality and supply, supplementary feeding etc, etc.

It is also important to consider how the environment in which your cattle are raised relates to how producers in your target market will raise their cattle. For example, if you are breeding bulls which will be used in large paddocks and in rough terrain then it would generally be preferrable if your production environment was the same. Similarly, if you are breeding cattle for grass finished systems and grain feeding them, then applying selection pressure to your herd for carcase productivity gains or fertility will likely result cattle not suited to your target market.

Technology enables much greater selection pressure to be applied. By using Artificial Insemination (AI) you can select for the best bull amongst thousands, and with Embryo Transfer (ET), the best cow amongst dozens.

A bull can leave hundreds or even thousands of progeny but a cow can only leave a few. Therefore, it is not only important but possible to apply very strong selection pressure on the bulls that you use. Progeny testing a bull’s many progeny gives us a very good idea of his genetic potential, whereas we have a much lesser idea on the cow’s genes because she only has a few calves that can be assessed. Genomic testing of a bull is expensive but spread over the many progeny of that bull, it can be cost effective.   If you use AI and select bulls that have very reliable data, you can place much less emphasis on selection in your females.