Red clover is an excellent feed for cattle, deer, young stock and fattening stock. It has, however caused some problems in breeding ewes due to its high oestrogen content. The oestrogen levels in red clover vary throughout the year, being high in spring and autumn but somewhat lower in summer. Insect damage and the silaging process can also markedly raise oestrogen levels. Red clover hay is considered safe as it is low in oestrogen due to the drying out process.
Ewes grazing pastures with a high oestrogen content will become temporarily infertile. While they will still cycle normally, there is a failure to conceive so many will be mated again in subsequent cycles. Conception from the first mating was lowered from 75% to 25% in one trial. Once taken off the affected pasture the ewes will return to normal fertility. For example, sheep grazing high oestrogen pastures in spring should be fully fertile by mating. The reason for the conception failure is due to a change in the composition of mucus secretion by the ewe’s reproductive tract. This mucus is responsible for sperm transport and when altered it becomes ineffective at carrying sperm to the unfertilised egg. The high oestrogens resulting from grazing red clover dominant pastures will also reduce the ewe’s ovulation rate, which results in less twins and triplets in sheep who do actually become pregnant. This becomes important when discussing treatment and management options.
Ewe’s grazing high oestrogen pastures for two or more grazing seasons may develop Permanent Clover Disease (PCD), which results in a degree of infertility from which the ewe does not recover. Other manifestations of PCD include:
- increased lambing difficulties due to abnormal dilation of the cervix and slowed uterine
- increased vaginal prolapses (bearings)
- milk production in maiden animals
- teat elongation and udder enlargement in wethers. Measuring teat elongation in wethers
was often used as a crude gauge of oestrogen content in pastures. Entire rams are
not affected by oestrogen levels in the pasture.
Androvax is a vaccine that stimulates the production of specific antibodies which change the hormone balance in the ewe and increase the number of eggs ovulated each cycle. This in turn increases the average number of lambs born per animal mated. From this definition it is clear that Androvax will only partly compensate for the type of infertility that ewes experience from grazing red clover, i.e. it will help increase ovulation rate but will not aid in sperm transport. By increasing ovulation rates it will however raise the lambing percentage of those ewes that do manage to conceive and therefore help put more lambs on the ground. Because some of the older ewes may have developed PCD it would make sense to use Androvax on young breeding stock, probably 2ths and 4ths. It would also be interesting to have a control group from this class of stock for a benchmark against which the success of the Androvax could be measured. This would be a group of unvaccinated 2ths and 4ths comprising around 10-15% of their total numbers.
Because of the excellent positive attributes of red clover which include: high yields, later flowering and high stock preference, cultivars have been developed with low oestrogen levels so these advantages can be fully utilised in sheep. Such cultivars include Grasslands G27 and, to a lesser extent, Grasslands Colenso.
Specific grazing management has also been shown to dramatically lower the effects of oestrogens on ewes around mating. This includes a system where ewes graze high oestrogen paddocks for a maximum of three days at a time. They must then be moved onto low oestrogen pasture for a minimum of four days before returning to the high oestrogen paddocks. This system is especially effective when red clover only makes up a small percentage of the sward.
It is possible to get samples from high risk pastures and have them tested for their oestrogen content. The cost is around $150 per sample and could be useful for finding out which paddocks are particularly unsuitable for breeding ewes, especially in autumn. Usually though, it is possible to determine which paddocks are high risk by knowing the composition of the sward and the cultivar of red clover used. A general rule is ewes grazing red clover will have there reproductive performance decreased to some degree and, if possible, breeding ewes should not graze these pastures for at least six weeks before going out with the ram.
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