Development of management strategies to improve aseasonal reproduction in sheep
Development of Management Strategies to Improve Out-of-Season Reproduction in Sheep.
Progress Report January 2006
Project Goals: To identify factors that impair out-of-season breeding in sheep.
Farm Update: We have increased the size of our flock slightly from 240 ewes at the onset of the project to our current number of 260 ewes. We continue to farm 120 acres but have leased an additional 60 acres this year to provide enough forage to compensate for the low forage yield due to drought last summer. Our management strategy for our flocks has not changed as we lamb three times per year (Jan-Feb, May-June and Sept-Oct) and ewes are given the opportunity to lamb every 8 months. Due to the mild winter we are currently experiencing (Jan 2006), we have been able to extend our winter grazing season longer than in past years.
Project Cooperators: The cooperators in this project are our own flock and another flock of nearly identical genetic background that is also on an accelerated lambing program. My family and I are providing the vast majority of labor necessary to execute this project. We took all measurements before and after the breeding season last spring and early summer, compiled data and also have performed several laboratory procedure on the samples obtained. The cooperator flock helped gathered together and moved sheep on the days we visited and also kept careful records during the subsequent fall lambing period.
Data Collected and Remaining Project Work: We weighed, recorded condition score and took blood samples before and after the spring breeding period in both flocks. We sampled 53 ewes in our flock and 59 in the cooperator flock. Ewes were carefully selected to try to match for age between flocks and ultrasound examination was used to screen study ewes to be sure that no ewes were pregnant just before the onset of the breeding season. Complete breeding soundness exams were performed on all rams used for breeding in both flocks 1 week before the onset of the breeding season. This exam was performed by the Theriogenoology Laboratory at Cornell University School of Veterinary Medicine. Fall lambing data on all project ewes in both flocks was recorded. Serum obtained from all ewes both before and after the spring breeding season was analyzed for leptin concentration. We are in the process of analyzing selenium status on plasma samples obtained on all ewes at the conclusion of the breeding season. The index of selenium status we are monitoring is plasma glutathione peroxidase (GPX-3) activity. We are awaiting results on serum ovine progressive pneumonia (OPP) titer. Once we have the final results for the remaining serum (OPP) and plasma (GPX-3) analyses we will compile all data and develop a statistical model to enable us to identify significant factors explaining out-of-season fertility.
Project Results and Accomplishments: Our preliminary findings are that both flocks displayed modest improvements in out-of-season fertility this fall compared to the previous 3 year average. Out-of-season fertility in our flock improved from 84% to 92% and in the cooperator flock, it increased from 21% to 30%. A large gulf remained however in out-of-season fertility between our flock, 92%, and our cooperator flock, 30%, which provides the model we were seeking to enable us to identify predictors for out-of-season fertility. Lambing rate (lambs born/pregnant ewe) were also quite different between our flock, 206%, and our cooperator flock, 128%. These large differences in both fertility and lambing rate cannot be explained by genetics as 90% of the ewes in our flock were either purchased from the cooperator flock or are direct descendants of these ewes. In addition, the rams used in both flocks are Finn x Dorset rams obtained from the exact same source flock. This indicates that profound environmental differences exist between these flocks that must explain the difference in fertility and lambing rate.
Our preliminary findings show striking differences in both ewe body condition score (2.0 cooperator, 2.5 our flock; scale 1-5, 1=thin, 5=fat) and weight (127 lb cooperator, 142 lb our flock) just prior to turning in the rams. Changes in body condition and body weight were also quite different between both flocks between the beginning and end of the breeding season. Body weight increased 11.2 lbs in the cooperator flock and 15.7 lbs in our flock and similarly, body condition score increased 0.2 in the cooperator flock and 0.7 in our flock. We suggest that the greater initial body reserves accompanied by the greater increase in body reserves over the breeding season in our flock accounts for a large amount of the difference observed in fertility and lambing rate between the two flocks. The results underscore the importance of adequate nutrition both prior to and during the breeding season in obtaining high fertility and lambing rate in sheep.
Our interpretation that adequate maternal body reserves at mating are a critical component of successful out-of-season mating is supported by our findings on serum leptin concentration in the two study flocks. Leptin is a hormone produced by fat and acts as a signal that regulates food intake, energy expenditure and reproduction. Leptin appears to be one of the signals that link the energy status of the animal to reproductive function. Leptin is also a readout of the energy status of the animal as it is positively related to body fatness and current energy intake. We found that serum leptin levels were elevated in the high fertility flock (3.6 ng/mL) relative to the low fertility flock (3.3 ng/mL) at the beginning of the breeding season and more perhaps more importantly, leptin concentration in the high fertility flock increased 10% whereas leptin concentration decreased 30% in the low fertility progress over the course of the breeding season. These changes in serum leptin are consistent with our findings on body condition score and weight change and further emphasize the importance of adequate nutrition in successful out-of-season mating.
The ram breeding soundness exams also revealed differences between both flocks which may also have contributed to the differences in out-of-season fertility observed. The rams in our flock had a much higher condition score (3.5) compared to the cooperator flock (2.2) and also exhibited higher ratings for several indices of semen quality (progressive motility, % normal sperm) and quantity (sperm concentration). All rams used produced viable semen however it is possible that the differences observed in general condition and semen quality and quantity were substantial enough to also contribute to differences observed in out-of-season fertility. The barriers to both male and female fertility are higher out-of-season than during the normal season and it seems plausible the ram breeding fitness observed in the cooperator flock was not adequate to overcome these barriers sufficiently enough to settle all the ewes in estrus.
We will soon have a complete data set as we await both OPP status data as well as selenium status data. Once we have this data, we will be in a position to determine whether or not these factors are important predictors of out-of-season fertility. Our complete data set will also allow us to perform a more rigorous evaluation of all the data to enable us to determine significant relationships and identify more decisively the important predictors of out-of-season fertility.
Conditions Affecting Project: We encountered one unanticipated condition that created a difference between the flocks being compared. The ewes in the cooperator flock were weaned approx 60 days prior to the beginning of the breeding season whereas the ewes in our flock were weaned approx. 10 days prior to the breeding season. If anything, this should have benefited the cooperator flock ewes as they had more time to regain condition following lactation. This however did not appear to have occurred as the cooperator ewes had lower body reserves than our ewes despite the longer recovery time. This difference prompted us to study an additional set of ewes in the cooperator flock that had a lambing to breeding season interval that was very similar to the ewes in our flock. The complexity with this group was that the lambs had not yet been weaned from these cooperator ewes when the rams were introduced. Only 2% of these ewes (1 out of 52) became pregnant. This is likely explained by the constraint imposed by lactational anestrous. In our flock, we have observed that ewes can overcome lactational anestrous during the fall but are less capable of doing so during the spring. In the end, we omitted the data obtained from the ewes that were still lactating during the breeding season because we felt that the ewes with the longer lambing to breeding season interval provided a more appropriate comparison group to the ewes in our flock.
Economic Findings: We anticipate that identity of plane of nutrition and possibly other factors as important factors in out-of-season fertility will allow the cooperator flock to develop a strategy to dramatically improve out-of-season fertility and therefore farm income. We will include an economic impact analysis in our final report.
Future Directions: A logical extension of this study would be to try to dissect the importance of male versus female fertility in out-of-season mating. To do this, we would utilize a pool of rams from our flock that all had high scores for male fertility to use in both flocks. We would also like to more rigorously examine the importance of prior versus current female nutrition during out-of-season breeding. To do the latter would require more controlled conditions than are commonly afforded in on-farm test situations.
Richard A. Ehrhardt, January 30, 2006
410 Slaterville Rd.
Dryden, NY 13053
Office Phone: 6078448367