Good nutrition is fundamental to ewe performance during all stages of the production cycle, with nutrient requirements changing throughout the year.
Dr Liz Genever, AHDB Beef and Lamb senior livestock scientist says: “In recent years, the sheep sector has gained a better understanding of the nutritional management of ewes. This has happened in tandem with an improvement in the genetic base of the national flock.
In late pregnancy (day 90-145) feeding plays a critical role in ensuring lambs fall within an acceptable weight range at birth. Optimum lamb birthweight for high survival is 4.2-7.4kg for singles and multiples. Birthweights below this range increase the risk of starvation of exposure for multiples and above this dystocia, mainly for singles.
Dr Genever says: “Low birthweight lambs tend to have lower glucose levels, reduced brown fat stores and increased surface area:weight ratio, all of which reduce their ability to keep warm and ‘get up and go’ at birth.”
“These lambs are slower to stand and suck less frequently than heavier lambs, reducing their colostrum intake, which is crucial to their survival.”
Body condition also plays a vital role during this period of pregnancy, as fitter ewes are able to mobilise their body reserves to sustain foetal and udder growth, according to Dr Genever.
“Where thin and fat ewes are fed the same diet, there is a positive correlation between the fat content of the lamb and the fat content of the ewe,” she says.
“This brown fat is critical to heat production and hence lamb survival. Ewes that mobilise less body fat during pregnancy produce lambs that stand and suck quicker and are more active over the first three days of life.”
In general, there are no benefits in lamb birthweight, lamb mortality or daily liveweight gain in lambs born to ewes that are over-fed in late pregnancy. Over-fat ewes tend to have a higher risk of prolapse and pregnancy toxaemia and they may experience a prolonged or difficult lambing due to having over-size lambs. They are then slower to groom their lambs, show reduced bonding behaviour, make fewer bleats and are more prone to rejecting their lambs. Lambs may also be less vigorous.
Somewhat surprisingly, thin ewes can also have excessively large lambs, as they partition nutrients to the foetuses and have a higher appetite than fitter ewes.
“Under-fed ewes have poorer maternal ability than those that are well-fed,” Dr Genever says.
“Behavioural impairments include taking longer to interact with their lambs, displaying more aggression to the lamb and spending less time grooming and more time eating, compared with well-fed ewes. This is thought to be due to a reduction in oestradiol which regulates the oxytocin receptors in the brain that are involved in the expression of maternal behaviour.”
Most of the mammary gland development takes place during the final weeks of pregnancy and there is a clear relationship between the ewe’s energy intake over this period and colostrum production.
Under-nutrition pre-lambing not only reduces the quantity of colostrum and milk produced, but also delays the onset of lactation and increases the viscosity of colostrum.
“Since viscosity and volume of colostrum are inversely related, this is a major issue for newborn lambs. The lamb may also find it more difficult to extract thick colostrum from the teat.”