The lower ovulation rate that occurs during the early part of the breeding season, September and October, has been found to be related to the shorter number of daylight hours in winter and early spring. To overcome this particular problem, a method of increasing the amount of daylight has been worked out.
Artificial lighting to increase ovulation rates early in the breeding season
Mares are placed under artificial light in mid-end of June and exposed to a total of 16 hours of continuous light a day. This extra light in winter causes stimulation of the receptor centres in the brain which in turn stimulates the production of two hormones. One of these, follicle stimulating hormone (FSH) stimulates the ovary to begin follicle production, while the second, luteinising hormone (LH), aids ripening and eventual ovulation.
This initiates the commencement of the first breeding cycle as early as July or at least early August, allowing all the problems encountered in September and October to be passed through before the breeding season commences.
In a given year, one stud using light treatment had 44 dry mares – 22 under lights and 22 without lights. Both were fed the same type of feed. The mares under lights all cycled normally early in August and, by the end of September, all of these were covered, with 14 being in foal by mid-October. Of the non-light mares, only 8 had been covered by the end of September and by mid-October only 5 were in foal. This has indicated the value of this procedure under Queensland conditions. It has been used successfully overseas for upward of 20 years.
Maximising breeding potential in maiden and dry mares
Maiden and dry mares often have long heat or oestrus cycles in September and early October, and the best solution to this problem is increased light in June through to October. However, this is not possible with every mare, so where a mare that has not been under light is in continuous heat, a condition known as ‘spring heat’, she becomes somewhat of a problem, particularly if ovarian palpation is not used to determine her exact state of follicle development. These mares are often covered by the stallion every second or third day for up to 10 or 20 services and even then frequently do not get in foal.
A treatment has become available for mares that have numerous small follicles (the follicle has to be between 30 and 35mm before it can be regarded as being mature) and have been in continuous oestrus for 15 or more days. This is a hormone compound that can be fed daily for 10 to 15 days. Its action is to temporarily suppress the action of natural pituitary hormones on the ovary. Once this suppression has occurred, the treatment stops and there is a new release of hormones, usually in a more balanced manner, and a normal follicle develops with a fertile egg becoming available approximately 4 to 10 days after the end of treatment. It is essential that problem mares be carefully examined and assessed before this treatment is used, as some conditions do not respond and disappointment results from inaccurate diagnosis and treatment.
Retained corpus luteum
Some mares show no signs of oestrus behaviour and examination of the ovaries may enable a decision regarding treatment to be made. It is possible for a mare to have ovulated quite a considerable time ago (before the onset of winter) and to have developed a normal corpus luteum (CL). The CL is the post-ovulation product in the ovary that fills the cavity left by the ovulating follicle. It secretes progesterone for the maintenance of pregnancy.
When this occurs, the mare will not cycle until the corpus luteum has regressed. Drugs are now available for the treatment of retained corpus luteum, but they are very powerful drugs and, if not administered correctly, can cause colic with severe discomfort to the mare. They can also cause abortion if the mare happens to be in foal. Used correctly, they are of great assistance during the breeding season.
Also, they are useful only for the retained corpus luteum and do not actually make a mare come into oestrus. Mares that are not in season may belong in this category or may be in winter anoestrus.
Anoestrus in mares
Mares in anoestrus can roughly be divided into these types:
- True anoestrus in which no follicles are present - the ovaries are small and quite inactive. Increasing daylight and adequate feed probably remain the best treatment at present. The use of a newer hormone preparation has given very limited success and is expensive with disappointing results.
- Shallow anoestrus - these mares respond poorly to teasing but do show some ovarian activity. Again, better feeding, longer hours of daylight, being teased regularly and being left near to a teaser for long periods often result in the mare starting her early oestrus cycle. These are often irregular and may not develop into a full oestrus with a fully developed follicle. However, these mares usually do cycle normally with the next heat period.
- False anoestrus - these are the ‘shy’ mares that may not show to the teaser but may show to other mares or geldings, or even only to the stallion when actually being covered. Examination of the ovaries, cervix and uterus often shows a mare in readiness for service, but actively rejecting the attentions of the teaser.
If a mature follicle is present and the cervix is relaxed, these mares may be covered. Only rarely do they reject the stallion. Care must be taken that the stallion is not placed in a position where he may be injured by such a mare during service.
Post-service feeding of the mare
Once the mare has ovulated, and if a fertile service has been given, it is important that the mares be fed sufficiently well to gradually increase their body condition. This does not mean that they should get fat, as this can be as detrimental as being poor.
It has been found that if the mare is on poor feed between 21 and 40 days after service, there is a high risk of either absorption of the foal or early abortion. Adequate feeding can help reduce this loss at this stage of pregnancy.