Excess dietary rumen degradable protein intake in ewes, its effect on ovulation rate, fertilization and embryo survival both in vivo and in vitro.
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The literature on the effects of dietary protein on fertility in ruminants is reviewed and it is concluded that failure to distinguish between rumen degradable and undegradable dietary protein may explain the equivocal nature of the published findings. Thus an experiment was designed in which ewes were used to determine whether or not high levels of rumen degradable protein affect ovulation rate, fertilization and embryo survival in vivo and subsequently during in vitro culture. In addition the effects of alterations in rumen degradable protein on plasma urea, ammonia, progesterone and luteinizing hormone were investigated. Thirty Greyface (Border Leicester x Scottish Blackface) ewes were randomly assigned to three treatments and given a basal control diet which met their energy requirements for body weight maintenance. One treatment involved supplementing the basal diet with 24 g of urea/day (low urea) and another with twice this amount (high urea). Both the low and high urea treatment diets exceeded the rumen degradable protein requirements (+ 56.5 g and + 125.5 g/day, respectively). The ewes received the diets throughout the experimental period. The ewes were put on the experimental diet around 60 days after lambing. Following a single injection of prostaglandin (Pgf2 alpha) each ewe received an intravaginal controlled internal drug release (CIDR) device which supplied exogenous progesterone for a period of 12 days. Ovulation was induced using pregnant mare serum gonadotrophin (PMSG) at the time of CIDR device removal. Plasma samples were taken just before CIDR device insertion and thereafter once daily. These were analysed for progesterone. In the period starting 24 hours after CIDR removal plasma samples were taken at 2 hour intervals for 32 hours and were analysed for luteinizing hormone (LH) to determine LH surge onset and amplitude. These samples were also analysed for plasma urea and ammonia over a time span of 24 hours. Intrauterine insemination (lUAI) using a laparoscopic technique was used to deposit approximately 100 x 10(6)spermatozoa in each uterine horn approximately 52 hours after CIDR device withdrawal. Embryo recovery was carried out surgically at 4 days and 11 days after lUAI, with half of the ewes from each treatment group having their embryos recovered on each day. Embryos were assessed using a stereomicroscope and where one or more was recovered one embryo was returned to the donor. Remaining embryos recovered at day 4 were cultured in vitro for 72 hours prior to a two hour incubation with phenylalanine. The embryos recovered on day 11 were only incubated in phenylalanine for two hours but were not cultured in vitro. There were no significant differences in ovulation rates which were 4.1 ± 0.92 in the control, 2.9 ± 0.50 in the low-urea and 2.4 ± 0.42 in the high-urea group. The mean percent embryo recovery rates were not affected by "day of recovery' and the overall values were 50.1 ± 10.90 in the control, 40.0 ± 12.4 in the low-urea and 24.7 ± 10.4 in the high-urea groups. At embryo recovery on day 4, two of ten embryos in the control, one of three in the low-urea and three of four in the high-urea groups were considered non-viable. There were more non-viable embryos from ewes in the high urea group (4/4) than the control (3/10) after 72 hours of in vitro culture. There was a positive correlation (r= 0.75, p< 0.001) between 3H-phenylalanine incorporation and stage of development of embryos following 72 hours of in vitro culture. More pregnancies were sustained in the ewes on the control diets (6/8) than on the high urea diet (1/3).Plasma urea levels were significantly affected by diet (Control 2 ±0.2, low urea 5 ± 0.3, high urea 7 ± 0.3 mmol/1 ; P < 0.01) and were elevated throughout the experiment in animals receiving urea supplements. Plasma ammonia levels in the high urea group showed transitory elevations of concentration after feeding and were significantly higher than the control and the low urea groups (P < 0.05) for 4 hours after each feed.There was no treatment effect on plasma progesterone concentration before, during or after CIDR device withdrawal. LH surge onset time and amplitude was not correlated to ovulation rate and was not affected by treatment.
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