Smaller ewes bring best returns: study

Farmers could lift production by putting more effort into their smaller or medium-framed ewes than into bigger ones, a Lincoln University study suggests.

Alastair Nicol, senior lecturer in animal science, said that in recent years the tonnage of lamb produced in New Zealand had been maintained in the face of a 20 per cent or more drop in the ewe population.

This had been done by increasing the net lambing percentage from 100 per cent to 119 per cent, and increasing the average carcase weight from 13.5kg to 16.5kg.

"It has been a great success story, but if a current producer is lambing at 175 per cent and the lambs are 16.5kg, where do they go for the next lift in production?"

The study was done with support from Meat and Wool Innovation, to see if total production could be raised by substituting smaller-framed ewes for heavier ewes.

A comparison of two flocks was made by running them side by side in a paddock. The pasture height across the whole paddock was kept uniform by adjusting the fence between the two groups.

"We found in both years the same number of big ewes needed 55 per cent of the paddock, and the smaller ewes 45 per cent. That's an approximate 18 per cent difference in stocking rate," Dr Nicol said.

"It turns out that between lambing and weaning they were actually carrying the same total liveweight per hectare."

Weaning weights were 2kg lighter in smaller ewes, and lambs on average took 14 days longer to reach slaughter weight.

Honours student Lisa Rutherford compared ovulation rate to liveweight. As expected she found the bigger ewes had a higher ovulation rate and therefore bigger potential for multiple births than smaller ewes.

"But it didn't seem quite as big as what we'd anticipated," Dr Nicol said.

The point at which ovulation rates peaked was at liveweights of about 68kg.

Dr Nicol said that when the upper limit of the liveweight-reproductive rate relationship had been reached, further increases in the number of lambs produced per hectare could come only from carrying more smaller ewes, increased genetic potential, or both.

- NZPA