Dead rams live again

11-01-2010 | Gerard Hall

It sounds like a movie script - the chance to breed from rams that have been dead for more than two decades.

Thanks to some smart thinking and a Rip van Winkle time capsule, researchers can show there has been a substantial change in the performance of the New Zealand sheep flock during the past 30 years.

There has been little change in lamb birth weights, but substantial progress has been made in increasing lamb growth rates, weaning weights, hogget live weights (LW) and fleece weight (FW).

Fortuitously straws of semen from seven Romney rams born in 1980 were placed into a stainless-steel time capsule. The capsule, which weighed about 100kg, contained liquid nitrogen, which kept the semen frozen.

Two years ago the Meat & Wool New Zealand Central Progeny Test (CPT) was fortunate to get hold of some of the semen. It had been kept because the rams, used early in the 1980s, were considered to be industry leaders at the time for growth and carcase traits.

Under tight security, the time capsule was flown south. The semen was thawed and inseminated into ewes at AgResearch's Woodlands farm as part of the 2007 CPT mating.

There has been much anecdotal discussion about whether today's NZ sheep model is better than its counterpart from 20 or 30 years ago. What has been lacking is proof that differences in animal performance have occurred over time. This study is filling in many pieces missing from the jigsaw.

The progeny of the 1980s rams have been run with others born in 2007 and their performance compared with those of the other dual-purpose rams used in the CPT.

Among the dual-purpose rams evaluated at Woodlands in 2007 were those representing the Romney, Coopworth, Perendale, Growbulk, TEFRom and Textra breeds or composites.

Also, as part of what is an on-going evaluation, the female progeny, now rising two-tooth ewes, have been retained to measure how they stack up as adult ewes, particularly in terms of lambing performance and mothering ability.

AbacusBio's Neville Jopson says the use of the 1980s sires is providing a snapshot of how far the country's flock has travelled genetically and where during the past 30 years.

Back in the 1980s there was some money in wool. The ideal lamb carcaseweight (CW) was 13-14kg and the amount of fat on the outside and within the carcase was not such an issue. With an eye muscle area (EMA) dimension of about 9cm, half the size of what they are today, loin chops were often referred to as resembling hockey sticks.

Jopson gave Country-Wide the first of what he describes as "then-and-now comparisons".

The report card makes interesting reading. Substantial increases have occurred in areas such as lamb growth rates and hogget LWs.

Ram breeders' emphasis on traits such as resistance/resilience to internal parasites, and meat traits including saleable meat yield, is more recent and there is still room for considerable improvement.

Other than at birth, progeny of the 1980s rams had significantly lower LWs at every measurement point, increasing from a 2.1kg advantage at weaning to a 7.7kg difference at 12 months old. Lamb birth weights were similar across the progeny of all rams.

To get an estimate of the actual genetic improvement that has occurred, Jopson says you need to double the differences.

For any animal, the father and mother each contribute half of their offspring's genes, so if the rams supply only half of the genetics, the difference in progeny performance has to be doubled to give the actual difference between the rams.

When shorn as hoggets (12 months old) the FWs of the modern-day sired progeny was also significantly higher. The extra 0.37kg is 20% higher than that of the 1980s progeny.

However, Jopson points out that most of the FW difference is likely to be due to the 15% difference in LW between the groups at the same age.

"If we can make the crude assumption that sheep are spherical in shape, then it is possible to estimate how much the wool growing surface would increase with a 15% increase in liveweight.

The heavier the sheep, the greater their surface area, and the more wool they can grow."

Interestingly there were no significant differences between the two groups when it came to measuring individual dag scores, bare belly and breech scores.

But the progeny of the 1980 sires had significantly lower faecal egg counts (FEC) than their modern-day mates. With just 59% of the eggs per gram at each of the two samplings, they were superior for resistance to internal parasites.

Jopson says these results are consistent with several other studies that have linked wool production and growth rate with parasite resistance.

Jopson's study also assessed changes that have occurred to several carcase traits. They include eye muscle area, fat depth over the eye muscle (C measurement), and dressing percentage and Alliance Group VIAScan traits, including the amount of lean weight in the hind leg, loin and shoulder.

These are the same traits that Alliance Group reports back to its farmer shareholders on their lamb killing sheets.

There were no significant differences in fat depth, but the eye muscle widths and depths were understandably larger among the modern-day sired progeny, by 6.9% and 5.1% respectively.

Comparisons of dressing out percentages (carcaseweight divided by pre-slaughter liveweight) and the three VIAScan measurements shows no significant change has occurred in either dressing out percentage or the amount of lean weight (yield of saleable meat) when expressed as a percentage of CW.

Jopson says that while the total weight of saleable meat in each carcase is considerably greater, it is simply due to the increase in lamb slaughter weights that has occurred over recent years rather than increasing the weight of meat per kilogram of carcase.

Meat quality traits assessed as part of the study included loin pH 24 hours after slaughter, loin fat and meat colour including the degrees of lightness, redness and yellowness.

Colour was assessed 24 hours after slaughter and after the meat had been left to "bloom" for a period of 30 minutes. When raw meat is cut and the surface is exposed to air for the first time, the meat "blooms", increasing in redness over a half-hour period.

Small but significant changes in pH were found between the 1980s sires and the other rams, with the pH levels of the 1980s sires 0.05 pH units lower.

Meat redness, an important component of meat colour, which consumers relate to quality and freshness, was 0.8units lower in the progeny of the modern-day sires.

It will be another year before the lambing performance of the 80s sired two-tooth daughters will be available.

How representative the original 1980s sires were of the industry remains an unknown, as is whether rams being evaluated in the CPT are average sires or industry leaders as some believe.

Even so, Jopson says the changes in performance are substantial and fit with industry genetic trends.

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