Under-nutrition in late pregnancy and early lactation can harm the ewes. Pregnancy toxaemia is also known as 'twin lamb disease' since it is more unlikely to affect ewes having twins than ewes bearing a single fetus. Milk fever is another fatal disease arising from deprivation of feed at lambing time. It is caused by low levels of calcium in the blood. If lactating ewes are yarded or otherwise suddenly deprived of feed shortly after lambing, they can become weak and collapse.
Recovery is rapid if affected ewes are injected with calcium borogluconate.
However, by the time symptoms are seen it is usually too late to treat them. The appetite of ewes increases enormously immediately after they give birth. There is a slow increase in the final stages of pregnancy, but the increase during lactation is much more dramatic. For maximum milk production, a ewe needs about 2.5 times the feed required to maintain a dry ewe. This feed requirement is rarely met for autumn and early-winter lambing flocks unless they receive high rates of supplementation.