C. Bailey, J. Gibbons, P. Melendez
Texas Tech University, School of Veterinary Medicine, Amarillo, TX 79106
Introduction
In the dairy industry, there has been considerable drive to maximize birthing rates and profitability by using genetics from beef cattle in both lower performing and lower genetic quality cows. This has allowed cows that are of average quality or aged genetics to provide added value to the operation in the form of crossbred calves for the beef industry.
One measure of infertility in multiparous cows is late embryonic mortality. Although there is likely a higher percentage of embryonic loss during early pregnancy (prior to Day 30; [1]), late embryonic mortality (LEM; defined as pregnancy loss that occurs between 30 and 60 days of gestation) is also a contributor to low profitability. Other research has shown that roughly 12.8% of dairy cows will undergo LEM between 28-42 days post-breeding, averaging a 0.85% loss per day [1]. Some of the factors that cause LEM include genetics, infectious diseases, poor prophylactic management, sanitation, and season of the year. However, genetic defects of the embryo may also account for up to 20% of early embryonic mortalities [2].
A common method of detecting pregnancy is by measuring Pregnancy Specific Protein B (PSPB) and plasma progesterone (P4) at specific times post insemination. An assay testing PSPB levels indicate a positive pregnancy if PSPB levels are a >10% above non-pregnant cow levels approximately 28 days post artificial insemination [3,4]. Elevated progesterone concentrations (>1 ng/ml) are also useful as a pregnancy detection aid, as they typically begin to taper off around 15 days post estrus in the non-pregnant cow but, are maintained in pregnant cows [5]. Both PSPB and P4 were evaluated and compared between groups in a subset of cows in this study.
The objective of this study was to compare the LEM of Holstein cows bred to Limousin bulls to Holstein cows bred to Holstein bulls. Optical density as a measure of PSPB and P4 concentrations were also evaluated from a blood sample collected at 30 days post breeding for comparison between a subset of 25 cows per group. Further, the LEM was evaluated based upon the time of year that the breeding occurred Summer (April 1 – September 30) and Non-Summer (October 1 – March 31).
Methods
The Limousin X Holstein and Holstein X Holstein breedings in this study were from a large dairy in the southeast US, which consisted of 12,847 Holstein cows. In this herd, 1,166 cows were dry with an average of 171 days open. The cows were milked 3 times a day with a rolling herd average of 14,600 kg of milk per year and were fed a total mixed ration based on corn silage, grass silage, and concentrates. The reproductive program consisted of a 60-day voluntary waiting period and timed artificial insemination using a variety of ovulation synchronization protocols. Cows were diagnosed for pregnancy status by trans-rectal ultrasound between 28 to 35 days post breeding. If the cow was diagnosed open, she was subjected to a resynchronization protocol. If the cow was diagnosed as pregnant, she was rechecked for pregnancy status between 50 to 57 days post breeding by rectal palpation. Blood was collected at random (n = 25 cows per group) at the time of ultrasound pregnancy diagnosis (28-35 days) and tested for PSPB levels expressed as optical densities and P4 concentrations measured by Radio-immuno Assay.
Results
Normal LEM for this herd is approximately 12%. Overall, for all cows bred in the summer the LEM was 15.2 ± 0.6% while those bred in the non-summer had an LEM of 9.9 ± 0.3%, (P<0.0001). Overall, for all Holstein X Holstein embryos the LEM was 15.2 ± 0.6% while the LEM for all Limousin X Holstein embryos was 9.8 ± 0.3%, (P<0.0001). Differences (P<0.05) in LEM between Holstein x Holstein and Limousin X Holstein were observed during the non-summer months (15.1 ± 0.8% versus 8.3 ± 0.3%, respectively) but not for the summer months (15.1 ± 0.9% and 15.3 ± 0.9%, respectively). summer = 15.23 +/- 0.6%, non-summer = 9.88 +/- 0.3% P<0.0001
In Figure 1, lactations within breed combination that do not share a common superscript are different (P<0.05). Further, there was a trend (P=0.08) for a difference between Lactation 2 and Lactation 3 for Limousin X Holstein embryos in the non-summer months and there was a breed combination difference (P<0.05) for all lactations during the non-summer months but not for any lactations during the summer months.
Effects of Season on LEM
Cows bred in the summer months (Figure 1), regardless of mating, had a higher LEM compared to cows bred in the non-summer months. A substantial difference in fertility was noted among cows carrying crossbred embryos especially during the non-summer months. Taken together, these data suggest that heat stress during the summer months combined with the stress of milk production, may affect the uterine environment specifically and these stressors cannot be overcome by altering the breed of the sire. Alternatively, in the non-summer months, the crossbred embryo apparently adapts more readily to the stress of milk production alone than do the Holstein X Holstein embryos.
Protein Specific Protein B and Progesterone
Optical densities of PSPB and P4 concentrations were compared between groups that were diagnosed pregnant at Day 30. The results showed a PSPB optical density of 4.44 in the Holstein X Holstein group and a 3.25 in the Limousin X Holstein group with a pooled standard error of ± 0.27 and a P-Value of 0.023. Progesterone concentrations were not different (P>0.05) between groups and were 8.66 ng/ml in the Holstein X Holstein group and 8.96 ng/ml in the Limousin X Holstein group with a pooled standard error of ± 0.57.
Conclusion
This field study demonstrated that breeding Holstein cows with Limousin semen reduced LEM by approximately 4% overall and there were seasonal and lactational differences in LEM. During the summer months, the breed combination had a lesser effect on LEM perhaps as all cows exhibited some form of heat stress; however, during the non-summer months, purebred Holstein embryos had a higher LEM than the Limousin X Holstein crossbred embryos, indicating inherent embryo loss dynamics due to factors other than genetics or lactation. These results indicated that breeding beef bulls to lower quality genetic Holstein cows decreased LEM regardless of time of year. However, the lower LEM was especially evident in early lactation cows bred during the non-summer months which likely contains cows that can still contribute genetically to the herd. The exact components of the “protective mechanism” associated with producing crossbred embryos may be due to hybrid vigor, but has not been fully elucidated; however, it seems to be mostly present during the non-summer months underscoring the ever-present stress of milk production on the females. The heat induced stress associated with the summer months historically [6] leads to an increase in LEM rates in Holstein embryos and was lactation dependent in both groups in this study. Progesterone concentrations for both groups were similar indicating that corpus luteum function was likely not related to LEM. Lower optical density which is reflective of the concentration of PSPB was seen in Holstein cows bred to Limousin bulls versus Holstein bulls. More research may be necessary to determine if PSPB plays a role in embryo viability during early pregnancy or if elevated optical density of PSPB is reflective of embryos in distress [7].
References
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