Understanding the basics of heritability – Part 1

This week we focus on sheep and goat farming, where genetic markers are now being used to select for production.

goat-farming-heritability
In small-scale systems genetic factors that increase survival are probably more important than increased production.
Photo: Cheryl McCrindle

It is important for anyone breeding livestock to understand the difference between genetic (inherited) and environmental (climate and management) factors in the phenotype of a farm animal (what it looks like). This will help even small-scale farmers.

A good example of a genetic factor influencing phenotype is the red-brown colour of a Boer goat’s head. This colouring is passed on through its genes to its offspring. By contrast, a white goat could have a brown head if it were kept in an area with plenty of red mud (environmental factor), but its offspring would not be born with brown heads.

‘Heritability’ measures what proportion of a factor (phenotypic trait) you observe in an animal is due to its genetic background and what is due to the environment.

The genomes of sheep and goats are currently being studied to find genetic markers linked to functional factors such as meat, wool and milk production, as well as disease resistance and fecundity (the number of offspring per animal per year).

Genetic markers are already being used in New Zealand to increase meat and wool production in sheep.

Environment
The environment will affect all inherited traits. If the genetic make-up of a sheep makes it possible to slaughter a lamb that weighs between 50kg and 60kg, remember that you still need to feed the sheep! In small-scale farming systems, the cost of feeding a sheep enough food for it to reach 50kg to 60kg before the two-tooth stage may be far more than you will be paid at slaughter.

Also, it will not help if the genetic make-up of a ewe allows it to give birth to triplets twice a year, as the cost of feeding and managing this type of ewe would be very high.

By contrast, small-scale, intensively managed goat milk dairies are becoming profitable in South Africa, so breeding for higher milk production and fecundity using genetic markers may become the norm.

Choosing traits
It is sensible to focus on genetic (heritable) traits suited to your farming system. In some communal systems, for example, your main problem may be stock theft and predation by stray dogs or jackal. Large, docile sheep will struggle to survive here, and you will do better with small, athletic goats with sharp horns that can thrive on thorn bush.

If these goats are kraaled at night and the heavily pregnant ones kept in the kraal so that they do not have to lamb in the bush, you will be guaranteed at least two lambs a year. Keeping the lambs in the kraal and feeding them lucerne or even cut thorn bush branches and deworming them at weaning, will give you a profit of about R1 000/goat/year.

At the other end of the scale are intensive small-scale systems with plenty of water where you can feedlot lambs and keep sheep on irrigated pasture. In this case, fecundity, milk production, rapid growth and parasite resistance are the heritable traits to look for.

This is the model in many European small-scale systems. Even students are employed to supervise the birth process in the ewes, weighing each new-born lamb hourly and making sure it drinks enough colostrum, then supplementing lambs until weaning.