By Dr Clive Dalton
What’s a Selection Index?
A Selection Index is used where you have a lot of different traits to consider, when deciding which are the best individuals in a group to use as parents for the next generation.
Using a Selection Index to identify top sheep was the start of a breeding revolution, not just in New Zealand but in sheep flocks worldwide. Prior to the development of an Index, breeders were more likely to use ‘Independent Culling Levels’ when a sheep had to be good in a whole range of traits to be selected for breeding. If a defect shoed up in a single trait, then it was culled!
It was like sitting for an overall Certificate which was made up of different subjects - and you had to pass them all. If you failed one test, then you failed the Certificate. In the old School Certificate – no matter how good you were at Geography and History, if you failed English and Maths you failed the lot. It was tough, as brilliance in one subject didn’t compensate for poor performance in another.
An Index allows balancing or compensation between the various component traits that go to make up the Index, so you end up with a ‘balanced’ overall measure of performance.
Tony Parker’s Romney flock
Tony Parker’s Wairunga Romney flock in Havelock North was thought to be the first flock in New Zealand to have a Selection Index calculated for it incorporating fertility (Number of lambs born NLB), growth (weaning weight WW) and fleece weight (FW). This was due to the work which Professor Al Rae from Massey and his students Graeme Hight and Neil Clarke did after Al’s return from Ph.D. studies in America.
Massey Cheviot flock
Dr George Wickham, formerly Seniour Lecturer in Massey University’s Sheep and Wool Department told me about some of his interesting memories. Here they are:
‘I doubt that the selection index for Tony Parker’s flock was the first one used in New Zealand. One very likely candidate was selection in the Cheviot flock at Massey (Massey College in those days). Al Rae split the flock into two sub flocks:
(1) A traditionally selected sub flock selected on visual appraisal.
(2) A sub flock selected on performance.
I was involved in collecting some of the data from the ‘stroppy little bastards’ when working as a junior research officer in the late 1950s, but I can’t remember the precise details of how they were selected or an Index calculated. I know that number of lambs born (NLB) and weaning weight (WW) were the main traits in this Index.
Some years later I did a comparison of the performance of the two sub flocks, and was surprised to find that there was no difference between them. After some further detective work, the farm staff told me that Bob Hewitt, the then sheep farm supervisor, had culled most of the rams that had been originally chosen for the performance-selected flock, saying their conformation wasn't good enough. He had replaced them with rams he thought ‘looked better’. Bob was a dyed-in-the-wool traditional breeder from a Romney stud background.
It’s also possible that Pat McMahon who was a student at Massey at the time did some calculations for an Index. Pat worked closely with a group of Romney stud breeders in the Karere area near Longburn. The Voss, Neilsen (of Romney N gene fame), Tanner, Mitchell and Buchanan flocks were all in this area.
The Vosses kept very detailed performance records and after doing his Masterate on the N-genetics of the Romney, Pat went to the wilds of Yorkshire and did a PhD with Speakman at the Wool Research Institute at Headingly in Leeds. When he returned to Massey in the late 30 - early 40s, Pat used the Voss flock records to calculate heritabilities of a number of traits and presented papers on sheep breeding at the Massey sheep farmers’ conference. Pat finished his career as Professor of Wool Science at Sydney University in the 1980s’.