Sheep investigated and data collection

Eight hundred forty-eight NZ Romney lambs, the progeny of 19 unrelated industry-sourced rams that were part of a progeny test on a commercial farm, were investigated. The gender, birth weight, birth rank (i.e., whether they were a single, twin, or triplet), and rearing rank were recorded for each lamb. All the lambs were weaned at ∼90 days of age, weighed, and separated based on gender into two mobs. The preweaning growth rate of the lambs was calculated as the average daily weight gain (grams/day) from birth to weaning.

As most of the female lambs were kept as ewe replacements for the larger commercial base flock, the draft weight and carcass data were only available from male lambs and a small number of cull ewe lambs. Lambs weighing >37 kg were first drafted for slaughter at around 16 weeks of age, with a second draft at ∼20 weeks of age. All remaining male lambs were slaughtered at ∼24 weeks of age. Draft weight and draft age were recorded for each lamb.

Hot carcass weights (HCWs) were measured directly on the processing chain (Alliance Food Limited, Smithfield, Timaru, NZ), which is the weight in kilograms of the carcass minus the head, gut, and pelt. Video image analysis (VIAScan; Sastek, Australia), developed by Meat and Livestock Australia and described by Hopkins et al. (2004), was used to estimate the following carcass traits: lean meat yield (expressed as a percentage of HCW) in the shoulder (shoulder yield), loin (loin yield) and leg (leg yield), and total yield (the sum of the shoulder, loin and leg yields for any given carcass), and V-GR (a VIAScan assessment of subcutaneous fat depth near the 12th rib). To describe the distribution of lean meat across the carcass, the proportion of total yield of shoulder, loin, or leg was also recorded, this being the yield of the specific part of the carcass divided by the total yield and expressed as a percentage.

At tailing, blood samples from all these sheep were collected onto TFN paper (Munktell Filter AB, Sweden) by nicking the lamb's ears and genomic DNA was then purified for PCR analysis using a two-step procedure described by Zhou et al. (2006).