Changes to Survival in NZGE: restricting lamb survival to lambs born to two-tooth and older ewes
A review of Survival, comparing data from lambs born to hoggets with survival data for lambs born to older ewes, revealed that using data from hoggets does not add value to the survival prediction of lambs born to ewes. Additionally, it introduces some bias to the survival analysis. The data for lambs born to hoggets has been used in the survival module since SIL’s inception.
A decision has been made to restrict survival information to data on lambs born to two-tooth ewes and older, beginning from the New Zealand Genetic Evaluation (NZGE) extracted on Friday 28 July.
Reasons for changing survival data
- A recent review indicated that survival of lambs to hoggets does not serve as a useful predictor of lamb survival to ewes.
- There are significant management and environmental differences between hoggets and ewes which impact the lamb survival analysis:
- Hoggets are immature and still in the growth phase of life.
- Mating weight, growth trajectory over the pregnancy, feed levels, and birth weights differ from those of lambs from older ewes.
- Hoggets often lamb at a different time than older ewes.
- The accuracy of hogget lambing data recording varies:
- It is not uncommon to see only singles, only twins, no dries, no deaths, or very high death rates amongst lambs born to ewe hoggets.
Overall response to removing ewe hogget data
Restricting the survival dataset to exclude survival data for lambs born to hoggets results in changes to some rams that have a lot of hogget data. Some changes are also observed for rams in flocks that do not practice hogget lambing but are connected in some way to rams that do.
For some rams, the changes are more significant, with a few rams experiencing a change of 500 cents for DPS, and a very small number changing by more than 1000 cents. For most flocks, the changes will be minor, and the overall dual purpose genetic trend graph remains very similar – see below.
Fig 1: Genetic Trend Graph for Survival in all Dual-Purpose Flocks
Blue line = current DPS,
Red line = hogget data excluded
Note: values will be a more accurate prediction of merit for Survival