It takes about three days for the embryo to negotiate the oviduct and enter the uterus. The embryo floats freely in the uterus and ‘hatches’ from its shell when it is about a week old. It attaches itself to the inner lining of the uterus between approximately 15 and 30 days into pregnancy. During this first month the embryos are very vulnerable, and about a quarter of them perish (Henderson, 1990). It is therefore important that during this first month of pregnancy ewes are kept on an adequate level of feeding. This is especially crucial on hill farms, where weather conditions are often atrocious during the mating period. Ewes should not be handled during this period, to avoid stress. Diseases that cause fever, such as tick-borne fever and liver fluke, may lead to embryonic losses. Selenium deficiency has recently been implicated in early embryonic loss.
To ensure maximum growth of the placenta and an adequate supply of milk after lambing, it is vital to feed ewes appropriately during mid-pregnancy. Ewes with a condition score of 2.5 (see MAFF, 1994) at mating should not be allowed to lose weight. Ewes with a condition score of 3 to 3.5 at mating are allowed to lose condition of up to half a condition score, but only if they have not lost condition during the first month after mating.
Many farmers have their ewes scanned during mid-pregnancy to determine the number of foetuses present, so that feeding can be appropriate during the latter stages of pregnancy. Furthermore, in extensive sheep production more attention can be focussed on ewes bearing multiple lambs (see FAWC, 1994). Suitable handling systems have to be set up to reduce the amount of stress during scanning (Baldock and Sibly, 1990; Baxter et al., 1983; Hargreaves and Hutson, 1990; Hutson, 1993). As most of the lamb growth occurs during the last eight weeks of pregnancy, the ewes have an increasing demand for energy, protein, vitamins and minerals. These have to be fed in a more concentrated form, as feed intake is reduced due to the space taken up by the lambs in the abdomen. As at least 60% of the dry matter intake in organic farming has to come from roughage (UKROFS, 2000), this has to be of high quality during the latter part of pregnancy. A deficiency in energy supply leads to pregnancy toxaemia. Sudden changes in the diet may lead to ruminal acidosis, which in turn may lead to pregnancy toxaemia and/or hypocalcaemia. Prolonged periods without food, during, for example, transport and handling may also predispose pregnancy toxaemia and/or hypocalcaemia. Overcrowding and inadequate trough space may lead to bullying and a reduced feed intake in some ewes. This also predisposes the animal to pregnancy toxaemia and/or hypocalcaemia. Copper poisoning due to a prolonged period of feeding concentrates high in copper and the use of the more susceptible European breeds is becoming an increasing problem in conventional sheep farming, but is rare in organic farming.
Foetal losses should be minimal during mid- and late-pregnancy. Abortion, however, may cause large losses to farmers. The four main causes of abortion are enzootic (chlamydial) abortion, toxoplasma abortion, campylobacter abortion and salmonella abortion.
An increasing number of conventional farmers on lowland, upland and hill farms are housing ewes, either over the whole period or for a restricted period around lambing (Slade and Stubbings, 1994). Organic farmers should aim to lamb outside, weather permitting. If a flock already suffers from infectious disease problems, then housing such a flock is likely to magnify the problems as the stocking rates are increased. Enzootic (chlamydial) abortion, campylobacter abortion, border disease, footrot, pasteurellosis, jaagsiekte, maedi visna, Johne's disease and keratoconjunctivitis are all important. Another problem, which has increased over recent years simultaneously with housing, is listeriosis caused by feeding poor quality silage. However, closer observation indoors makes some problems, such as prolapse, acidosis, pregnancy toxaemia and hypocalcaemia, easier to detect and treat.
Recently, it has become more common to shear housed ewes in winter. This reduces heat stress (see MAFF, 1995) and has been shown to increase birthweights of lambs. However, whilst reducing the risk of heat stress, winter shearing will make the ewe more prone to cold stress and energy deficiency in adverse weather conditions.