Measuring Genetic Merit

Measuring and monitoring are key to our goal of constantly improving our sheep genetics. All stud animals are evaluated for genetic merit through – Sheep Improvement Ltd (SIL).  SIL is New Zealand’s genetic engine provided by Beef + Lamb New Zealand Genetics. It generates the estimated breeding values for different traits and the economic indexes.

NZ Maternal Worth (NZMW)

The NZMW Index is the sum of all the most commonly measured dual purpose production trait indexes (Growth, Survival, Adult Size, Reproduction and Wool) added together.

‍Below is a simple explanation of how the NZMW index helps sheep breeders select the animals that will provide the greatest economic benefit to their flock.

Indexes are an economic value expressed in cents compared with an average stud sheep in 1995.

Purchasing a ram with a NZMW of 2000 means a sheep breeder has purchased a ram with genetics of $20 greater value than a ram with a NZMW of 0. For example, if the genetic merit of a commercial sheep flock is on average NZMW 500 (i.e. $5.00 greater than stud sheep in 1995) and the farmer buys a ram with a NZMW of 2000: this ram will pass on half of his genetics to his progeny ($10.00). The other half of his progeny’s genetics will come from the dam ($5.00). Consequently the progeny will be have a NZMW of 1250 or $12.50.

Assuming both rams are of equal health, eczema tolerance and soundness. A ram buyer can quickly assess the cost benefit of two different rams by looking at their NZMWindex.

An Example of Comparing Rams using the NZMW Index

A ram with a NZMW of 2500 compared with another ram with a NZMW of 1500 has $10.00 greater value. While this doesn’t seem a lot, when that ram goes across 100 ewes lambing at 140% each year for 3 years minimum, then he sires 420 progeny.  Because each of his progeny gain half of their sire’s genetics, they gain a genetic merit $5.00 greater than if sired by the poorer ram with a NZMW of 1500.

So the extra return on this ram is $2,100 greater than the poorer ram with a NZMW of 1500 over 3 years.

i.e. 420 progeny x $5 = $2,100

When considering the price of rams, and assuming both rams are equal in structure and longevity, paying even up to a $1000 more for the high index ram compared with the low index ram, will result in a net return of $1,100.

The real advantage is that the improved genetics, when included into a breeders ewe flock, are cumulative with each generation. As the ewe also passes on the improved genetics. This is why buying top genetics is one of the best investments a sheep farmer can make. Research into the most profitable farmers shows that they always purchase better quality rams and bulls than average performing farmers.

When looking at various ram breeders to purchase from, along with the usual assessment of flock structure and hardiness always ask for their NZMW – indexes to benchmark them between other breeders. But be careful to compare apples with apples or more specifically indexes with indexes.

Make Accurate Comparisons

As a quick reminder, the core traits that make up the NZMW index are:
- Growth, Adult Size, Reproduction, Survival and Wool.
However many breeders like us measure more traits than these core traits. In our case we measure Meat, Worm Resistance and Facial Eczema also and each of these have index values. Many breeders will add these extra indexes onto the NZMW index to provide a fuller picture of the animal at ram selling time. An example is NZMW+M (meat), or NZMW+X (eczema) which can easily add another 1500 index points to the NZMW index. This practice is fine, but can often confuse some clients into thinking they are purchasing a ram in the top 5% nationally on NZMW at say 3500, yet it is actually just a 2000 NZMW index ram, with an additional 1500 index points from Eczema and Meat. While these are important, their inclusion and how they are weighted affects the index value, so ensure you are benchmarking like with like between breeder catalogues.