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Are All Heat Periods the Same?

Dairy Pipeline: May 1995

by Ray Nebel
Extension Dairy Scientist, Reproduction

Many dairy producers are using a prostaglandin product (Lutalyse or Estrumate ) to short cycle or induce cows to come into heat. Until recently prostaglandins were only used at herdcheck when the veterinarian palpated a corpus luteum (CL) present on the ovary of a cow open beyond the desired breeding time or 50 plus days in milk. Prostaglandins precipitate the regression or death of a functional CL. The CL is the structure on the ovary that produces the hormone progesterone and prevents the release of other reproductive hormones which allows follicles to grow and mature for the next estrus and ovulation. So, progesterone controls the cycle and prostaglandin can short circuit the normal process and short cycle the cow if a CL is present on one of the ovaries. Recently, many dairy producers are using prostaglandin products in a Target Breeding or Monday Morning Breeding Program to induce cows that have not been seen in heat or bred within a particular time (60 days for example). If a group of cows that have not been palpated are given a prostaglandin product only those with a CL will have a chance to come into heat. On average 7 of 10 cows given prostaglandin should respond and have a chance to express estrus. Not every cow that responds to prostaglandin will show heat, so a good thumb rule is that 60% or 6 out of every 10 cows that are administered prostaglandin (not identified as having a CL by palpation) should be observed in heat 2 to 5 days after administration.

We have studied cows given Lutalyse and have found no difference in the estrus and ovulation characteristics relative to natural cycling cows. The standing heat period (time from first to last mount) was approximately 8 hours and consisted of approximately 8 mounts (the cow in heat received an average of 8 mounts, but ranged from 1 to 85) . Heifers have more mounts and the duration of heat is slightly longer. Ovulation or release of the ovum from the follicle occurs at the same time for prostaglandin induced and naturally occurring heats, 5.4 hours. This means that breeding times should not be changed for cows induced or short cycled with a prostaglandin. The prostaglandin products, Lutalyse and Estrumate, are important tools to be used to effectively observe cows in heat and maintain desired calving intervals and these products do not have negative effects on estrus characteristics or conception rates if the cows are bred after being observed in heat.

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