Excessive heat affects the fertility of high production lactating cows, and reduced pregnancy rates (PR) are observed during summer and early fall. Embryo production programs are used to produce more calves from high genetic merit animals, but could it also increase fertility by bypassing all the negative variables affecting the embryo development before Day 7 (oocyte development, ovulation, fertilization, early embryo development)? The data from AIs and embryo transfers (ET) between June 2017 and May 2019 were analysed. June, July, August, September, and October were called critical months (first-service AI conception rate dropped below 44%). The cows were located at Maddox Dairy in Riverdale, CA, USA, a Holstein herd that milks 3500 cows with a 305-day mature-equivalent milk production of 12 800 kg. First- and second-lactation cows were enrolled in a Presynch-Ovsynch oestrus-synchronization program and scheduled for the first AI at 86 days after calving or to receive an embryo 7 or 8 days after the expected heat. The embryos were produced invivo or invitro from Holstein donors and were transferred fresh or frozen. Blood was sampled on Day 30 after expected heat day (23 days after embryo transfer), and pregnancy was detected by the IDEXX PAG Bovine Pregnancy Test. Table 1 summarises the results, where ET PR% is the number of pregnant cows divided by the number of cows that received and embryo. All the cows synchronized for AI were bred, but only cows with the presence of a corpus luteum (CL) on ET day received an embryo. The presence of a CL was not detected in 28.7% (471/1642) of the cows (32.2% in the critical months and 25.7% in the others). Unfortunately, we could not detect the presence of a CL by ultrasonography every time we transferred embryos, so the nonovulation rate might be overestimated. The cows without a CL were considered open and used to calculate the adjusted PR (AdjPR%). Embryo transfer PR is superior to that of AI, especially during the critical months. Fresh invivo embryos have the most impact. When the cows without CLs are considered open, the difference between AI and ET is still evident for fresh invivo embryos. Besides producing animals with higher genetic merit, depending on the type of embryo used, ET can increase fertility in lactating Holstein cows, especially during the critical months. The other benefit of using ET is that cows that do not ovulate are synchronized right away, which is not the case for AI cows.
Table 1.AI×embryo transfer in lactating Holstein cows1 from June 2017 to May 20192
Item
Critical months (June to October)
Other months (November to May)
All year
%PR
n
Adj PR%
n
%PR
n
Adj PR%
n
%PR
n
Adj PR%
n
Artificial insemination
41.2%
896
41.2%
896
47.7%
1767
47.7%
1767
45.5%
2663
2663
Fresh invivo embryo
62.7%
373
47.5%
493
69.5%
262
55.3%
329
65.5%
635
50.6%
822
Frozen invivo embryo
59.3%
221
44.8%
292
59.4%
256
47.3%
322
59.3%
477
46.1%
614
IVF fresh embryo
47.9%
167
36.2%
221
54.0%
363
43.0%
456
52.1%
530
40.8%
677
Total embryos
58.5%
761
44.2%
1006
60.2%
881
47.9%
1107
59.4%
1642
46.1%
2113
1Lactating Holstein cows, first and second lactation, first service, Presynch-Ovsynch, 85 DIM.
2PR%=the number of pregnant cows divided by the number of cows that received and embryo; AdjPR%=adjusted pregnancy rate.
Successful embryo transfer from Hokkaido native pony after artificial insemination with frozen semen
