Speaking at two Beef + Lamb New Zealand Farming for Profit field days in Canterbury, Massey-based veterinarian Kate Griffiths outlined the results of a six-year trial which looked at a ewe’s ability to survive as a productive animal until she is culled-for-age.
Up until now, there has been very little information gathered about on-farm ewe mortality, with previous estimates ranging from between 2.8 and 15.7 per cent.
The trial followed 13,142 ewe lambs from before their first breeding through until they were culled for age as six-year-olds. The ewe lambs, which were all electronically tagged, were run on commercial hill country farms in the North Island. They were predominately Romneys with some composites.
Over the length of the study, losses varied considerably from 3.5 per cent to 24.8 per cent per annum and varied from year-to-year, but Kate also pointed out that this was only one age-group within the wider flock.
“These dead or missing ewes represent a huge area of loss,” says Kate.
Most years, mortality sat between 8 per cent and 12 per cent and the highest mortality rates were in the younger and older ewes.
During the course of the trial, a huge amount of data was collected about these sheep. They were all weighed and body condition scored at pre-mating, scanning, set-stocking and weaning and where possible, reproduction performance was also recorded, along with the date and reason for culling.
While they tried to collect tags from ewes that died on farm, this proved very challenging and Kate said they did not get close to recovering all the tags from ewes that had presumably died.
This was one of the main limitations of the study.
Post-mortems carried out on thin ewes – with a body condition score of two and below - showed that the majority died without any significant disease and may well have responded to an intervention such as priority feeding.
The losses were tracked throughout the study with the first lot occurring when dry hoggets were culled (85 per cent of ewe lambs were mated) and this was followed by hogget mortality which ranged between 4 per cent and 10 per cent.
“We are losing a lot of hoggets before they even get into the mature ewe flock.”
A subset of data looking specifically at mated and pregnant hoggets highlighted that success in rearing a lamb was influenced by body weight, body condition score and weight changes during pregnancy. In essence, it all came down to feeding.
As hogget weights at set-stocking reduce, the risk of wet/dries increases dramatically and this relationship was the same across all flocks.
What did vary across flocks was the optimum weight.
But as a rule of thumb, the heavier the hogget is at set-stocking the more likely she is to rear a lamb through to weaning.
A number of hoggets in the trial lost weight between scanning and set-stocking and this massively increased their risk of being wet/dry.
“Weight is important so the hogget needs to continue to grow by 100-150gms/day throughout the pregnancy.
“For every 1kg gained between scanning and set-stocking, there is a 10 per cent reduction in the likelihood of the hogget being wet/dry.”
Kate says ewe longevity is significant factor affecting the economics of the breeding ewe flock.
To maintain ewe numbers, replacements are needed but these replacements have higher management costs and are a less flexible stock class.
High attrition rates mean fewer ewes are going to a terminal sire and means farmers cannot apply a high degree of selection pressure to their hoggets before they enter the mixed-age flock.
There is also a biosecurity risk associated with buying-in replacements.
The costs associated with on-farm mortality is high as there is no cull value and if the ewe is pregnant, the loss of lambs also needs to be taken into account.
As reproductive performance tends to increase with age, there is a potential production cost associated with a flock with a high proportion of younger ewes.
This study was funded by Beef + Lamb New Zealand, Massey University and The C.Alma Baker Trust.
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