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Effects of Daylight on heat, breeding, foaling...
Effects of Daylight on heat, breeding, foaling...


There was an interesting article just published describing the effect increased daylight hours have on the pregnant mare.  Of course we have talked about the use of extended hours of light on the open mare.  16 hours of light every day starting December 1 or earlier, will usually bring the  open mare into heat 30 days earlier in the spring. This work tested the same light conditions on the pregnant mare.

To summarize the work, increased hours of light resulted in births as much as ten days early.  The theory being that the mare exposed to light sleeps less and spends some of those hours awake eating.  This leads to a heavier foal and an earlier birth. The weight of the foal is important in the triggering mechanism that starts delivery.  That is, unless the mare has not been removed from fescue grass or hay.   Fescue reduces the mare’s ability to recognize a term foal which can allow it to become one of the ‘giant’ foals.

From our experiences this year so far, I speculate the mild winter we enjoyed had a similar effect on the delivery dates.  Perhaps the reduced maintenance requirements of the warmer weather allowed more of the foodstuffs to be sent to the foal, causing it to grow faster.  Many of our foals are coming early.  Fortunately, if the mares were removed from fescue at least two months before the due date, they will go ahead and develop an udder and be ready with an adequate supply of colostrum.  The normal signs of foaling are present.  The mare’s udder fills and becomes distended.  Her vulva swells, and the muscles on either side of the tail relax.  They are just there earlier than we expected.

As we mentioned in our previous article, many of the mares are also cycling.  We usually expect 75% of the mares to go into anestrus (a period of time when the ovaries are not functioning) during the winter.  The increased daylight hours after December 21 (equinox, and no relation to the horse) stimulate the mare’s brain to start sending signals to the ovaries.  At this time the ovaries will often be no larger then a walnut.  After about a month of this the ovaries start to stir.  Starting from rest, several follicles will start growing over the next month.  Unfortunately, many are growing at the same speed.  After another month, or what is usually March, this first crop of follicles starts to reach the surface of the ovaries.

The mare starts to feel the effects of the hormones from the follicles.  The reproductive tract will become active, edematous, and swollen, and the mare will be in heat.  The problem is that there are several follicles having the same effect on the poor mare.  Of course this situation is even more frustrating to the owner, and eventually even to the stallion if breeding is attempted.  The first follicle will make the mare receptive to the stallion for a few days.  She may then have a day of quiet before the next follicle matures, keeping her in heat for a few more days.  If she is being bred every other day, even this activity begins to grow old.

The longest period of continuous heat I can remember reported to me was 45 days.  If that mare had been presented to us earlier, we could have shortened the cycle with hormones.  The  heat from these multiple follicles is what we call a transitional heat.  This heat is rarely fertile as the eggs have “aged” during the quiet period and are no longer capable of conception.  So we consider the first, or transitional, heat to be non-fertile.  However, once this transitional heat is over, the next one is usually 21 days later and lasts for about five days.

As we know, only when the heats occur at regular intervals (three weeks) and last for a regular time period (an average of 5 days) should we attempt breeding.  When the mare has an irregular heat or recycles too soon, some other unusual event occurs:  the chances of conception drop rapidly.

We have seen mares this week in their transitional heats.  They were in heat and their ovaries had multiple follicles on them, all about the same size.  For them to overcome the transitional heat, all one needs is patience.  Once they go out of heat, the next one will be fertile and we can proceed with our breeding plans.

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