Studies at New Zealand’s Massey University showed that shearing a twin-bearing ewe at mid-pregnancy can increase lamb birth weights, weaning weights, and survival rates.
Paul Kenyon, a recent Doctorate recipient, found that twin or triplet lambs born from ewes that were shorn 88 days after mating had up to 1.5 lbs. greater birth weight, up to 2.2 lbs. more weaning weight, and survivability was 3% higher. The ewes also grew as much as 8.8 oz. of extra wool for the year.
Dr. Kenyon said that a lot of the multiples born in New Zealand are below the optimum birth weight, which is from 8.8 to 12 lbs. He said many multiples are less than 8.8 lbs. The “cold stress response” induced by the mid-pregnancy shearing makes the ewe metabolize some of her body fat reserves, generating more available energy for the unborn lambs.
The greater birth weights are not due to the increased intake, he said, but “we think that when the animal is cold it increases production of the thyroid hormone thyroxine, speeding up its metabolism and breaking down its body reserves for itself. Because the ewe is getting increased energy from its fat, glucose is being spared so there is more glucose available to the fetus.”
A similar response was demonstrated over 50 years ago in swine in Kansas, in which sows were given little shelter (a 16-foot high wall toward the wind) but were fed 16% more feed. The extra feed in those tests produced greater birth weights, but only when fed to spare-sheltered sows, not to well-housed ones.
The New Zealand research found that the method works only with their multiple-bearing ewes. “It’s best suited for highly fecund flocks or in situations where farmers can manage multiple and single bearing ewes separately. We don’t want to make singles better, as there is no extra benefit and may in fact cause lamb losses.”
In a trial with no birth weight response, the flock was mainly single-bearing ewes. It actually wound up costing extra, because of the second shearing in one year.
The technique is mainly suited to large-scale farming operations. In Europe, where most sheep are housed during pregnancy, the shearing takes place closer to lambing time, to cut down on the stress of excess warmth in the ewes, thus contributing to a heavier lamb. In New Zealand, the purpose of mid-pregnancy shearing is opposite: To induce cold stress response.
Shearing 88 days after the ram is first introduced means the ewes will be between 50 days to 88 days pregnant, with day 70 being the actual optimum. Shearing at that time should enhance growth of the placenta, which should improve birth weights, says Dr. Kenyon.
The ewes need to be kept well fed in order to obtain the birth weight response says Dr. Kenyon, “The ewes need to be in good condition to start and if they don’t get enough feed during pregnancy they will use their body reserves to keep themselves going and won’t have anything left to put into improved fetal growth.”
He said shearing with a cover (snow) comb still brings about a birth weight response, but gives the ewes some protection against the weather. He said they’re using the method mostly on New Zealand’s milder North Island, but that it could be used in the colder South. “Ewes [in colder climates] should be provided with shelter after shearing.” The University team wants to look into whether blade shearing (using hand shears) will give the same effect.