Progress 01/01/02 to 12/31/04
Outputs
A series of studies were conducted to determine the effects of timing of bovine somatotropin treatment in relation to estrus on subsequent pregnancy rate and fetal growth. Treatment of cows maintained on endophyte-infected fescue pasture with somatotropin two days prior to expected estrus resulted in a slight increase in pregnancy rate. Treatment of lactating cows with somatotropin at insemination also resulted in a slight but non significant increase in pregnancy rate. A seasonal effect was also observed for somatotropin treatment. Treatment of heifers at estrus or 5 days after estrus resulted in increased pregnancy rates during cool weather, but decreased pregnancy rates in hot weather. No differences in pregnancy rate were observed in cows that were treated with somatotropin at insemination versus delaying treatment to 14 days after insemination. A study evaluated effect of somatotropin treatment on pregnancy rate, corpus luteum function and fetal growth in heifers
when given at estrus, 14 days after estrus or both. Somatotropin treatment at insemination had no effect on pregnancy rate whereas treatment on day 14 or on both insemination and day 14 reduced pregnancy rate. Regardless of timing of treatment, somatotropin had no effect on serum progesterone or fetal growth on day 45, as measured by crown-rump length. While other studies have reported that somatotropin treatment increases the length of elongating embryos on day 17, these results suggest this increased growth does not translate in increased fetal size by day 45 of gestation.
Impacts
The extensive use of bST by the dairy industry, to increase milk production, can have both positive and negative consequences on herd fertility. Our studies should lead to better understanding of the role of bST in reproduction and should lead to development of procedures to avoid any adverse effects on fertility.
Publications
- Rorie, R.W., Rosenkrans, C.F. and Aishman, A.J. 2004. Effects of bovine somatotropin treatment on the AI pregnancy rate in dairy heifers. Reprod. Fertil. Develop. 16:132.
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Progress 01/01/03 to 12/31/03
Outputs
A study investigated the effect of bST (Posilac) treatment given at and/or after artificial insemination on subsequent pregnancy rate in dairy heifers. Also investigated, was the effect of bST on fetal size at 45 days of gestation. Estrus was induced in crossbred dairy heifers by a single or repeated (at 14-day intervals) injections of PGF2alpha. Heifers were observed two or more times daily and artificially inseminated about 12 hours after detection of estrus. At estrus, heifers were randomly assigned across treatments in a 2x2 factorial arrangement. Treatments were bST injection at either the time of insemination (Day 0), on Day 14, or at both insemination and on Day 14. Untreated animals served as controls. Blood samples were collected on Day 14 for progesterone analysis, since bST has been reported to stimulate corpus luteum growth and steroidogenesis. On Day 45 after estrus, ultrasonography was used to determine pregnancy status and measure fetal-crown rump
length. Treatment of heifers with bST at insemination had no effect on pregnancy rate when compared with the control group. However, bST treatment on Day 14, or on both Day 0 and 14, reduced pregnancy rate, when compared with the control and Day 0 bST treatment groups. Pregnancy rate was similar for heifers receiving bST on Day 14 versus Day 0 and 14. Treatment of heifers with bST on Day 0 had no effect on serum progesterone on Day 14. Fetal growth, as measured by crown-rump length, was unaffected by treatment. In contrast to studies with lactating dairy cows, bST treatment at insemination does not appear to increase pregnancy rates in dairy heifers. This could be due to inherent differences in the fertility of lactating dairy cows versus heifers. The reduction in pregnancy rates of heifers treated with bST on Day 14 or Day 0 and 14 is in agreement with previous studies. Further study is needed to determine why bST treatment 1.5 to 2 weeks after estrus is detrimental embryo/fetal
survival. While bST treatment has been reported to increase the length of elongating embryos on Day 17, this increased growth does not appear to translate in increased fetal size by Day 45 of gestation.
Impacts
The extensive use of bST by the dairy industry, to increase milk production, can have both positive and negative consequences on herd fertility. Our studies should lead to better understanding of the role of bST in reproduction and should lead to development of procedures to avoid any adverse effects on fertility.
Publications
- No publications reported this period
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Progress 01/01/02 to 12/31/02
Outputs
Initial studies have been conducted to evaluate the effects of bovine somatotropin (bST) treatment on the fertility of beef and dairy cattle. One study evaluated the effects of bST treatment on pregnancy rate in beef cattle maintained on endophyte-infected fescue. Although there were not enough animals in each treatment (n =11 to 15) for statistical significance, bST treatment two days prior to insemination resulted in a numerically higher pregnancy rate for cows on endophyte-infected fescue (26.7 vs. 14.3 percent for bST and control groups, respectively). The low, overall pregnancy rate was likely due to the use of timed inseminations. This study will be repeated, using a larger group of animals and inseminations based on observed estrus, to further evaluate the effect of bST treatment on pregnancy rate in cows exposed to fescue toxins. Another study was conducted with lactating beef cows to determine the effect of bST treatment at breeding on AI pregnancy rates. All
cows (n = 35) were artificially inseminated 12 hours after onset of estrus. Half of the cows received an injection of bST at the time of insemination. Pregnancy rate for the treated cows was 73.7 compared with 68.8 percent for the controls. A study was conducted to determine the effect of bST treatment at or after breeding on pregnancy rate in dairy heifers. Heifers (n=91) were assigned to a control group, bST treatment at insemination (bST 0), or bST treatment 5 days after insemination (bST 5). The study was conducted in two replicates, of approximately 45 animals each. The first replicate was conducted during May and early June (moderate temperature), and the second during July and August (hot temperature). Pregnancy rates for heifers in the first replicate were 92.9, 73.3 and 62.5 percent for the bST 0, bST 5 and control groups, respectively. Pregnancy rates for heifers in the second replicate were 57.1, 57.1 and 75.0 percent for the bST 0, bST 5 and control groups, respectively.
These results suggest that bST treatment increased pregnancy rate under conditions of moderate temperature, but decreased pregnancy rate during hot weather. A study has also been initiated to determine the effects of bST treatment on fertility of lactating dairy cows. At about 50 days of lactation, the cows were treated with Lutalyse. Half of the cows detected in estrus were treated with bST at insemination and the other half served as controls. The cows not detected in estrus were injected with GnRH 14 days later. Seven days after GnRH treatment, the cows were given an injection of Lutalyse and two days later, another injection of GnRH. All cows were inseminated 26 hours after the second GnRH injection. Half of the cows were treated with bST at insemination. None of the cows were receiving bST treatment when the study started. After the initial bST injection, cows continued to receive a bST every two weeks, throughout lactation. Pregnancy data is not yet available from this study.
Impacts
Growth hormone directly, or indirectly through the actions of insulin growth factor I, has been shown to have a role in numerous aspects of reproduction ranging from folliculogenesis/oogenesis to maternal pregnancy recognition and establishment. These studies should lead to better understanding of the role of growth hormone in reproduction and could lead to methods for improving productivity of cattle.
Publications
- No publications reported this period
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