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July 9, 2024

February 2024

Each year, Mark helps to select the ewe and wether lamb replacements at Glenthorne Station, Canterbury, New Zealand. In a recent video, he shared his approach to the selection process. Here we look at some of the factors Mark considers when classing lambs on a merino-based operation in New Zealand’s high country.

Selecting ewe replacements: the science and the art 

At Glenthorne Station, there are two ewe flocks; one is mated to merino rams and the other to terminals. Mark inspects the ewe lambs destined for the main merino ewe flock up to three times before their first mating. When selecting ewe lamb replacements, Mark’s priority is to look for “... our first-class ewes, which have got a good balance of wool, carcass and general type.” They also need the constitution to maintain their body condition through pregnancy and lactation in this challenging environment. Any ewe with a slightly off-wool type but good carcass attributes will join the terminal flock. 

Science plays a crucial role at neXtgen Agri, underpinning a raft of day-to-day decisions we support clients to make across their farming businesses. Several years ago, Glenthorne chose a new source of merino rams, switching from more traditional horned merinos to polled rams with more carcass and other desirable traits for this high country property. Mark points out, “We use the science pretty hard on the ram selection. So we make sure we're getting those carcass genetics and those worm genetics and those footrot genetics in the rams.” 

Mark acknowledges that there is some subjectivity in applying the science to selecting replacements, with a need for rapid decisions about the direction each animal will go. But twin-born lambs are identified, so they are not penalised when compared with the singles, to help the future reproductive performance of the flock. Other environmental factors that have influenced how the lambs look, such as the age of their mothers and the quality of the feed they have been on, are also taken into account.

A focus on wool in the wethers

Of the 2,800 merino wether lambs passing through the yards this year, only 800 or so will join the wether flock out on the hill. Mark highlights, “They’ve been selected for their wool-cutting ability and wool quality.” Well-nourished, fine (17-18 micron) wool, without excessive wrinkle, is the priority in this flock. While doing ability is important out on the hill, the wethers will not have the demands of pregnancy or lactation to contend with.

This focus on wool production means that single-born wethers are more desirable than twins in this instance. “Those single-born wethers are going to be slightly finer and cut more wool for their life. So, we're preferentially selecting singles for the wether flock and obviously looking for wool quality and wool cut and enough constitution,” explains Mark.

Mark outlines how ewes carrying a single lamb provide better uterine nutrition. From around day 90, when secondary follicles are developing in the skin of the fetus, single lambs develop more follicles than those born as twins. This results in lower micron and higher fleece weights as an adult. Nourishment of the wool is also a crucial factor in keeping dust out of the fleece when they are exposed to strong winds on the hill. 

Winter finishing and selling lambs store 

Lamb finishing and the sale of store lambs provide additional income streams at Glenthorne. Mark points out, “Income from surplus livestock sales is probably going to be over half of the income on a lot of farms - even though they might consider themselves as wool-producing farms - they’re still a big chunk of their income unless they're into the specialty wools.” 

Breeding for growth and carcass is important for increasing the value these animals add to the business. The bigger lambs, which are closest to finishing, will stay at Glenthorne through the winter, be shorn and then processed as part of a premium lamb programme. Cropping farmers on the Canterbury plains buy the lighter lambs store, freeing up feed for the stock remaining at Glenthorne. Mark highlights the importance of delivering “... a product that those finishing farms get a lot of value out of and therefore line up for again next year.”  

Mark explains: “We've bolted in a lot of good genetics into the Glenthorne lambs in terms of their growth and carcass. And so, once they're given an opportunity, those lambs, they'll hum. So a bit of a win-win: people picking them up for good value because they haven't got a lot of weight this year but then they'll turn into money.” He adds, “If you bolt in that carcass and that growth, in addition to the wool value, it's an animal that people are going to want to come back and buy.”  

Locating the sweet spot: merino vs terminal

A common question Mark gets is: “How many merino ewes do I need to mate to a merino ram versus how many can I mate to a terminal to produce that lamb that is closer to finishing and is a much more desirable product?” With terminal-sired lambs weaning at heavier weights and having more growth potential, there is a case for shifting the balance between merino and terminal matings. 

Mark points out, “What you’re trading off is selection pressure versus saleability of product and income from that product.” There needs to be a sufficient number of merino lambs to choose suitable replacements from each year, with enough selection pressure on the key traits in the breeding objective. On the other hand, “If you generate a heap of extras, then you’ve got a lot of product - a lot of animals - that aren’t as desirable for those finishing farms.”

Although it remains common to mate only 15 or 20 per cent of merino ewes to a terminal ram on many properties, in Mark’s opinion that can be increased. This means fewer cull merino lambs to shift off the property each season and more of the faster-finishing terminal-sired lambs to sell at heavier weights. Depending on the number of lambs being weaned and the degree of selection pressure required at a particular time, Mark observes that a 60:40 or 50:50 split between merino versus terminal matings works for many producers. 

But it depends on what stage a flock is at, explains Mark. “A lot of people are making a transition where you're trying to shift from a particular sheep type or you’ve changed genetics - and so you want the selection pressure, you want to keep heaps of females around so you can select the best types, the ones that picked up the attributes from the old that you wanted and from the new that you shifted for. But once you’re through that - and at Glenthorne we’re four or five years through that transition - you're starting to get a much more even type that you’re happy with, so the selection pressure isn't as hot.” 

Environmental effect

A tougher season, with less quality feed available in some areas of the farm, highlights the effect that environmental factors have on the expression of genetics. Nutrition, ewe age and the number of lambs per ewe all influence how a particular lamb fulfils its genetic potential. 

Instead of choosing only the biggest lambs with the shiniest fleeces on the day, Mark is mindful of why a lamb looks good today and why a lamb from another mob is not as well-grown or well-nourished in the wool. The aim is to strike a balance that provides the best chance for the desired genetics to flow through to future generations.

Mark reflects, “The thing to drive home, time and time again, is what the environment does to genetics. We all think that what we're seeing is what we get or that it's just an outplay of genetics. But once you see a few different mobs, a few different nutritional environments and you know the age of the mum that these lambs are all from and where they're running, it's such a powerful thing.”

Watch the full video on neXtgen Agri’s YouTube channel

Would you like to chat with the neXtgen team about how we can help unlock the potential of your farming business? Send us an email at

Dr Mark Ferguson
Article by:
Dr Mark Ferguson

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