Maximizing Litter Size − Timing When to Inseminate

Small litter size is most commonly caused by inseminating at the wrong time.  One would think that since sperm can live in the female vaginal tract for up to one week, and eggs are fertilizable for at least a four day period, that breeding close to the right date would be fine; however, that is not the case.  

Many new breeding managers do not realize that a bitch’s breeding behavior does not always coincide with the right time to mate.  This is because standing and flagging, willingness to accept a male, the color of the vaginal discharge (thinning from blood to thinner), and the change in the vaginal smear are all influenced by estrogen.  Estrogen is a hormone that readies the uterus, vagina, and behavior of the bitch, but it is NOT the hormone that controls when the eggs ovulate.

The important hormone that triggers ovulation is called Luteinizing Hormone (LH).  After the eggs are released, cells in the crater-like space in the ovary where the egg was release a very important hormone called Progesterone.  

Eggs are most fertile 3 to 5 days after ovulation occurs; however, a bitch may exhibit prime mating behavior many days earlier or later than the peak time for fertilizing the eggs.  Insemination at the wrong time can result in low conception rates or small litter sizes.  If insemination is too early, there are fewer live sperm when the eggs are ready to be fertilized.  If insemination is too late, the eggs may have disintegrated due to being too old or the cervix may have closed.

Combining progesterone testing with LH testing is an ideal method for selecting the prime breeding date.  A spike in LH occurs 48 hours prior to ovulation. There is only one LH surge and it only last 24 hours.  Dogs produce immature oocytes at ovulation.  Once ovulated, these oocytes require 48 hours to mature enough to handle fertilization by sperm.  Therefore, if we know the date of ovulation, we can choose the optimal breeding dates.  

On the chart below, the pink spike is the rise in LH.  It can be measured by a blood test run in-house by using a test kit available from Synbiotics, CA, (http://www.synbiotics.com/Products/Canine_Reproduction.html).   The test kits cost about $20.00 US per test and have a short shelf life.

The blue line on the chart below is the progesterone level as it is measured every other day.   As you can see, the LH rise tends to be around the time the progesterone value jumps from <1.5 ng/ml to >2 ng/ml.  For countries using nmol/l instead of ng/ml, nmol/l can be easily converted to ng/ml by simply multiplying by 0.315.  Keep in mind that the exact value may vary from dog to dog, and some interpretation must be done.  

By running progesterone levels at a laboratory every other day, the increase in progesterone can be monitored.  Initial progesterone values at a brood’s heat start typically should be <0.2ng/ml.  If you get a value <0.2ng/ml, you can be assured that the brood is early in her heat.  Since in some bitches, the progesterone value at

the start of the heat is higher than <0.2ng/ml, it is important to obtain a blood sample within the first 48 hours of heat start to have a baseline number on that particular bitch.  

If the value is >0.2ng/ml, verify that her vaginal cytology is also early (<50% cornification).  Values under 1.2 ng/ml are typically still too early for a LH surge, especially if the blood from two days prior was 0.8.  But if her blood from two days prior was only 0.2, a surge is possible at the low value of 1.2.   When you see a brood that has a value of 1.5 or above, you should plan to do an LH test.