LORI.equine

Lactation and foaling

Keywords: milk, electrolytes, pH, foaling, equine

The first evidence that foaling is about to occur is a sudden and remarkable increase in filling of the mammary gland. In the illustration below, there is a remarkable difference between the mammary gland in the inset (approximately 1 week before foaling) and the mammary gland in the main image, one day prior to foaling. The larger inset serves as reminder that milk electrolyte or pH monitoring should begin when sudden enlargement of the mammary gland is noticed. Sampling is usually done once a day, usually in the evening.

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Sodium (and chloride) milk in colostrum decrease with the approach of foaling. At the same time, potassium and calcium concentrations rise. However, of the three electrolytes in question, it is calcium that shows the most acute change as foaling approaches. Around foaling, calcium concentrations are about ten times higher than those in blood. Therefore colostral calcium concentrations have been used for many years to indicate when foaling may occur.

Water hardness test strips as well as commercial tests are base on calcium concentrations but high calcium concentration alone are not highly correlated to the time of foaling. As a result, these tests are best suited as negative tests for foaling i.e. to indicate that foaling is unlikely to occur.
The image below shows the results of a common water hardness test superimposed on a graph of milk electrolytes.

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When the concentrations of  calcium, sodium and potassium are combined, one is afforded a better prediction of foaling than with calcium alone. A chart such as that shown here can be used to predict the spontaneous time of foaling and if necessary, when it is safe to induce foaling.

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The pH of milk has been examined and compared to milk electrolytes for its value in predicting the time of foaling. In preliminary observations, it appears that the pH of milk is significantly correlated to changes in milk electrolytes and may also be used to predict foaling. It is not clear at this time if pH is superior to milk electrolytes to predict foaling. It is however, very simple to determine pH and even pH test strips are sufficiently accurate for this purpose. Sampling is also done once per day, preferably in the evening. The reason for a drop in pH with the approach of foaling is not clear.

In the image below. the pH of milk is compared to the time of foaling, superimposed on a graph of milk electrolytes. The greenish bar indicates the approach of foaling.

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Colostrum also starts to become opaque (above) shortly before foaling because of an increase in content of kappa casein. This has also been used as an indicator of impending foaling. However little data exists on the accuracy of this observation. It is certainly not reliable in the hands of this author.

Note: "Waxing" (the accumulation of dried colostrum on teat tips) is often mentioned as a harbinger of foaling but many mares never show obvious waxing.

A note on the induction of foaling. One should not induce foaling in the absence of proof of readiness because premature foals are likely to experience malposture and neonatal respiratory embarrassment. Also, immunoglobulins increase in the mammary gland at an almost logarithmic rate as foaling approaches. Therefore a few days short of the natural duration of pregnancy may translate into a significant deficiency of immunoglobulins in the colostrum. For that reason and because of gut immaturity in the foal, failure of passive transfer is common when foaling is induced. One must only induce foaling when spontaneous foaling itself in imminent. Estimating the time of normal foaling can be difficult. This is important from the point of view of routine monitoring of foaling. It is however, absolutely critical when one induces foaling for conditions such as neonatal isoerythrolysis, ventral abdominal ruptures, orthopedic injuries etc. Again, one should let a mare foal spontaneously if at all possible; foal survival rates are not only higher than otherwise but should a complication arise, it is readily interpreted as veterinary malpractice.

References: 

Canisso, I.F. et al. 2013 Decreasing pH of mammary gland secretions is associated with parturition and is correlated with electrolyte concentrations in prefoaling mares. Vet. Record 173:218. Online ISSN 2042-7670

Ousey, J.C. et al.1984.  Preliminary studies  of  mammary secretions  in the mare to assess foetal readiness for birth. Equine  vet. J. 16: 259-263