Can Milk pH Testing Predict When a Mare Will Foal?

By Dr Duncan Houston

Waiting for a mare to foal is a special form of sleep deprivation. You check the camera, she eats hay. You check again, she lies down. You panic, she gets up and poos. Lovely.

Milk pH testing can make that final stretch much more manageable. It is a simple, low-cost way to help predict when a mare is getting close to foaling, especially when used alongside udder changes, waxing, relaxation around the tailhead, behaviour, and proper foaling monitoring.

The important word is help. Milk pH testing is useful, but it is not a magic crystal ball. It can reduce guesswork, but it should not replace observation, foaling alarms, cameras, or an emergency plan.

Quick Answer

Milk pH testing can help predict foaling because a mare’s mammary secretions usually become more acidic shortly before birth. A drop to pH 6.4 or lower is strongly associated with foaling soon, while a pH above 6.4 makes foaling within the next 24 hours less likely. In one study, using a pH cutoff of 6.4 had a positive predictive value of 97.9% for foaling within 72 hours and a negative predictive value of 99.4% for not foaling within 24 hours. (PubMed)

Why Predicting Foaling Time Matters

Most mares foal normally, but when things go wrong, the window for useful help can be short. Dystocia, red bag delivery, failure of the foal to breathe, poor colostrum intake, and retained placenta are all problems where timing matters.

Foaling prediction helps you decide when to:

• Increase night checks

• Watch the camera more closely

• Keep the mare in the foaling area

• Have someone physically available

• Prepare the foaling kit

• Arrange help for high-risk mares

• Reduce unnecessary nights of staring at a perfectly relaxed mare doing absolutely nothing

Colorado State University notes that mares commonly show variable pre-foaling signs, and about 70% of mares wax up before foaling, but the timing from waxing to birth can still vary. Milk calcium and pH testing can therefore be helpful, especially on smaller farms with limited staff. (Vet Med & Biosciences College)

What Is Milk pH Testing?

Milk pH testing measures the acidity of a mare’s pre-foaling mammary secretions.

In the week before foaling, mare milk pH is often around 7.0 to 7.4. As foaling approaches, the pH commonly drops. A pH of 6.4 or lower is a key threshold many breeders and veterinarians use as a sign that foaling is getting close. (Vet Med & Biosciences College)

Technically, in the days before birth, you are often testing mammary gland secretions or pre-colostrum rather than mature milk. For owners, “milk pH testing” is the practical term, but it is worth remembering that this fluid is valuable. Do not strip large amounts from the mare just to test.

Why Does pH Drop Before Foaling?

As the mare approaches parturition, the composition of the mammary secretions changes. Calcium and potassium tend to rise, while sodium and chloride fall. These electrolyte shifts are associated with the drop in pH.

A 2024 study on periparturient mares found that mammary gland secretions, electrolytes, and pH are used to detect impending parturition, and that mares showed sodium-potassium inversion and acidic pH at the time of foaling. (MDPI)

That is why pH testing can be useful. It is not reading the mare’s mood. It is detecting biochemical changes that commonly occur as the body gets ready for birth.

How Accurate Is Milk pH Testing?

Milk pH testing is one of the more practical foaling prediction tools available to owners, but accuracy depends on technique, strip quality, timing, mare factors, and whether the pregnancy is normal.

A 2013 study comparing pH, refractometry, and calcium testing in preparturient mammary secretions of mares found that using a pH cutoff of 6.4 gave a positive predictive value of 97.9% for foaling within 72 hours and a negative predictive value of 99.4% for not foaling within 24 hours. (PubMed)

That means the test is especially useful for this question:

“Can I be reasonably confident she probably will not foal tonight?”

It is often better at helping rule out immediate foaling than guaranteeing the exact hour of birth.

What Do the pH Numbers Mean?

The trend matters. A mare that drops from 7.4 to 6.6 is more interesting than a single random 6.8 reading with no history.

Colorado State University states that a drop in milk pH to 6.4 or lower is associated with foaling within the subsequent 24 hours, and that pH test strips calibrated in the range of 6.0 to 8.0 can be used as an alternative to a commercial pH meter. (Vet Med & Biosciences College)

What Supplies Do You Need?

You do not need a laboratory setup. You need simple, accurate tools.

Useful supplies include:

• Narrow-range pH strips, ideally around 5.5 to 8.0 or 6.0 to 8.0

• Strips with small increments, preferably 0.2 to 0.3 pH units

• Clean gloves

• Clean small collection container or direct strip testing method

• Good lighting

• Recording sheet or phone notes

• Foaling camera or alarm if available

Avoid broad-range aquarium-style strips that jump from one whole pH unit to the next. They are often too blunt for this job. The difference between 6.8 and 6.4 matters.

How To Test Milk pH in a Mare

Use the gentlest method possible. The aim is to collect a very small sample, not strip out colostrum.

1. Wash or sanitise your hands.

2. Wear clean gloves if available.

3. Make sure the mare is calm and safe to handle.

4. Gently clean any obvious dirt from the teat area.

5. Express only a tiny drop of secretion.

6. Apply the drop to the pH strip.

7. Read the strip at the manufacturer’s recommended time.

8. Record the number, date, and time.

9. Compare it with previous readings.

10. Do not keep milking the mare repeatedly if you already have enough to test.

Testing is usually done once daily as the mare gets close, then sometimes twice daily once the pH begins to fall. In research settings, mares have been monitored daily from around day 320 of gestation until foaling. (MDPI)

When Should You Start Testing?

A practical approach is to start testing when the mare is close enough that foaling is plausible and there is enough mammary secretion to collect safely.

For many mares, that means:

• Around day 310 to 320 of gestation

• Once the udder is developing

• Once waxing or secretion begins

• Earlier if the mare has an uncertain breeding date and is showing clear pre-foaling changes

Do not panic if there is no milk to test. Some mares, especially maiden mares, may not give useful secretions until very close to foaling. Do not aggressively strip the udder trying to force a result.

Milk pH Testing vs Calcium Testing

Milk calcium testing has been used for years to predict foaling. As mares get close, calcium in mammary secretions rises. Calcium tests can be useful, but they often require more sample handling, dilution, timing, and interpretation.

Milk pH testing is attractive because it is cheap, quick, and easy to repeat. Colorado State University notes that milk calcium testing is useful for predicting when mares are likely or unlikely to foal, while milk pH often decreases markedly within the 24-hour period before foaling. (Vet Med & Biosciences College)

In practice:

• Calcium testing can be useful and has a long track record.

• pH testing is often simpler for owners.

• Using both can improve confidence.

• Neither replaces watching the mare.

Foaling is not a maths exam. The mare is allowed to be annoying.

Severity and Risk Framework

Low Risk: pH Still High and Mare Looks Normal

The mare has a pH around 7.0 to 7.4, no major udder dripping, normal appetite, no concerning discharge, and no active labour signs.

Action: continue routine monitoring and record daily readings.

Medium Risk: pH Is Trending Down

The pH has moved from high 7s toward 6.8 or 6.6, the udder is full, the teats may be waxing, and the mare is showing mild behavioural changes.

Action: increase observation. Make sure the foaling area, foaling kit, camera, and emergency plan are ready.

High Risk: pH Is 6.4 or Lower

The mare’s pH has dropped to 6.4 or lower, especially if this is a clear change from previous readings.

Action: assume foaling may be close. Begin active foal watch, especially overnight. Do not rely only on the number.

Critical Risk: Labour or Emergency Signs Are Present

The mare is actively straining, the water has broken, a red bag appears, the foal is not progressing, or the mare is distressed.

Action: stop thinking about pH. This is now a foaling management or emergency situation. Call your vet immediately if anything is abnormal.

When Milk pH Testing Can Be Misleading

Milk pH testing is useful, but it is not perfect.

It can be less reliable when:

• The mare is maiden and does not show classic signs

• There is little or no secretion to test

• The sample is contaminated

• Broad-range strips are used

• The strip is read too late or in poor lighting

• The mare has placentitis

• The mare is running milk early

• The pregnancy is high risk

• The breeding date is uncertain

• The mare has unusual mammary development

• Testing is done inconsistently

A 2024 study found that pH can show some variation depending on storage temperature and time, although the clinical effect may be limited in many practical situations. This still supports the basic rule: test fresh if possible and keep your method consistent. (MDPI)

What Else Predicts Foaling?

Milk pH is only one piece of the puzzle.

Other signs that a mare may be close include:

• Udder filling

• Teat distension

• Waxing

• Milk dripping

• Relaxation around the tailhead

• Lengthening and softening of the vulva

• Restlessness

• Getting up and down

• Sweating

• Looking at the flank

• Reduced appetite

• Isolation from other horses

The problem is that these signs are variable. Colorado State University notes that waxing can occur 24 to 48 hours before foaling, but not all mares wax up, and the timing can vary. (Vet Med & Biosciences College)

That is why pH testing is useful. It gives you one more objective data point when the physical signs are being vague, which mares are absolute professionals at.

What Is Normal Foaling Progress?

Once foaling starts, pH testing no longer matters. Timing and progression matter.

Normal active delivery should move quickly. Merck Veterinary Manual describes Stage 2, the actual delivery of the foal, as usually taking 5 to 15 minutes unless there are complications. Stage 3, passage of the fetal membranes, should occur within 3 hours. (Merck Veterinary Manual)

A normal foal is commonly assessed using the 1-2-3 rule:

• Stand within 1 hour

• Nurse within 2 hours

• Mare passes placenta within 3 hours

AAEP’s foaling monitor sheet also uses the 1-2-3 post-foaling rule and includes IgG tracking for passive transfer. (AAEP)

When Is This an Emergency?

Milk pH testing is not for emergencies. It is for prediction. Once emergency signs appear, the pH number is irrelevant.

Call your vet immediately if:

• A red, velvety membrane appears instead of a white translucent membrane

• The mare’s water breaks and there is no clear progress

• The foal is not delivered promptly once active labour begins

• Only one leg appears

• The head does not appear

• The soles of the foal’s hooves point upward

• The mare strains hard without progress

• The mare is severely painful, collapsed, or bleeding heavily

• The foal is not breathing

• The foal cannot stand or nurse

• The placenta has not passed within 3 hours

• The mare rejects or attacks the foal

Colorado State University describes red bag delivery as an emergency where the normal grey-white amnion is not what appears first, and the foal’s oxygen supply may be compromised. (Vet Med & Biosciences College)

What Should You Do Next?

If your mare is approaching her foaling window:

1. Start recording daily observations. Note udder size, waxing, behaviour, appetite, vulval relaxation, and pH.

2. Use narrow-range pH strips. Broad-range strips are too vague.

3. Test at the same time each day. Evening testing is practical for many farms.

4. Watch the trend. A falling pH is more useful than one isolated result.

5. Increase monitoring as pH approaches 6.8. Have your camera, foaling kit, and contacts ready.

6. Begin close foal watch at 6.4 or below. Especially if the drop is sudden.

7. Do not overmilk the mare. Colostrum is for the foal.

8. Do not ignore labour signs because the pH is “not low enough.” The mare did not read the spreadsheet.

9. Have a vet plan before foaling starts. Emergency numbers should be ready before midnight drama begins.

10. Arrange post-foaling checks. The foal still needs colostrum intake, IgG assessment, navel care, and normal newborn milestones monitored.

Common Mistakes Owners Make

Trusting the Strip More Than the Mare

If the mare is actively foaling, the pH result does not matter. Watch the delivery, not the colour chart.

Using the Wrong pH Strips

Broad-range strips can make 6.4 and 7.0 look too similar. Use narrow-range strips with small increments.

Reading the Strip Too Late

Some strips change colour over time. Read them according to the instructions.

Testing in Bad Light

Colour interpretation in poor lighting is asking for trouble. Use bright, consistent light.

Stripping Too Much Colostrum

You only need a tiny sample. Repeatedly milking the mare can waste valuable colostrum.

Assuming pH Predicts a Safe Foaling

Milk pH may help predict timing. It does not tell you whether the foal is positioned correctly, whether a red bag will occur, or whether the newborn will nurse properly.

Forgetting Maiden Mares Are Sneaky

Maiden mares may show less obvious udder development, less waxing, and less predictable signs. They deserve closer observation, not less.

Can Milk pH Testing Prevent Foaling Problems?

Not directly. Milk pH testing does not prevent dystocia, red bag delivery, failure of passive transfer, retained placenta, or a weak foal.

What it can do is improve your timing.

That means you are more likely to be awake, nearby, and prepared when foaling happens. That is where the value is. In foaling, being present at the right moment can make a huge difference.

Prevention and Planning Checklist

Before the mare is due:

• Confirm the expected foaling date

• Prepare a clean foaling stall or safe paddock

• Set up a camera or foaling alarm

• Buy narrow-range pH strips

• Prepare a foaling kit

• Save your vet’s emergency number

• Check whether a Caslick’s needs to be opened

• Confirm vaccination timing with your vet

• Make sure transport is available

• Know what red bag looks like

• Know the 1-2-3 foal rule

• Plan a neonatal foal exam and IgG test

• Save the placenta for veterinary inspection

Milk pH testing works best when it is part of a system. The strip is helpful. The plan is what saves you when the mare decides 2:17 am is a lovely time to create chaos.

FAQ

What pH means a mare is close to foaling?

A milk pH of 6.4 or lower is commonly used as a strong sign that foaling may be close. It should be interpreted with the mare’s udder changes, behaviour, gestation length, and previous pH trend.

Does pH 6.4 mean the mare will foal tonight?

Not always. It means foaling is more likely soon, often within the next 24 to 72 hours. Some mares do not follow the textbook exactly.

Does a high pH mean I can stop watching?

No. A pH above 6.4 makes foaling in the next 24 hours less likely, but it does not replace monitoring. This is especially true for maiden mares, high-risk pregnancies, mares with abnormal discharge, or mares showing labour signs.

Should I test milk pH or calcium?

Both can be useful. Calcium testing has been used for many years, while pH testing is often simpler and cheaper. Using both can improve confidence, but neither replaces observation.

Can I use normal pH strips?

You should use narrow-range pH strips designed to show small changes around the important range, ideally around 5.5 to 8.0 or 6.0 to 8.0. Broad-range strips are often too hard to interpret accurately.

Final Thoughts

Milk pH testing is one of the most useful low-cost tools for predicting when a mare is getting close to foaling. A drop toward 6.4 or lower should get your attention, especially when the trend is clear and the mare is showing other pre-foaling signs.

The best way to use pH testing is not as a standalone answer. Use it as part of a calm foaling plan: watch the mare, track the trend, know the emergency signs, have your foaling kit ready, and know when to call your vet.

The pH strip helps you predict timing. Your preparation is what protects the mare and foal.

If your mare is getting close to foaling and you are unsure how to interpret milk pH results, udder changes, waxing, or early labour signs, ASK A VET™ can help you work through the next steps with practical veterinary guidance.