Does nutrition have an effect on the incidence of bearings in sheep?
Updated: Jul 5, 2020
Vaginal prolapses (or bearings) are a common condition affecting breeding ewes. The majority of prolapses occur in the 3 weeks prior to lambing. The yearly proportion of ewes affected can range between farms from 0.5% up to 10% of ewes.
The losses associated with bearings occur directly through ewe and lamb deaths, shepherd time and costs associated with treatment and premature culling of recovered ewes.
Some causes of prolapses that have been speculated previously include; multiple lambs, sloping terrain, previous dystocia, excessive fatness, lack of exercise and full bladders. The published evidence supports that multiple lambs, slope and previous dystocia were associated with the incidence of prolapses.
For this weeks summary we have looked at possible risk factors of vaginal prolapses and then looked at a paper that examines injectable vitamins on the effect of prolapses. The aim of that study was to determine if vitamin treatment reduced the incidence of vaginal prolapse in pregnant sheep on a farm that had high incidences of prolapses (>2%).
The risk of a prolapse occurring was 5.31 times higher for ewes carrying twins and 11.3 times higher for ewes carrying triplets, compared with single-bearing ewes.
If swedes were fed in late pregnancy then the ewes had 1.45 times greater risk of prolapse than ewes that were not fed swedes.
There was no association between body condition score and vaginal prolapses.
Injectable Vitamins A, D and E to pregnant ewes reduced the incidence of vaginal prolapse.
What is a vaginal prolapse?
Prolapses of the vagina occur as an accident in pregnant and non-pregnant females in many species. In sheep it is a disease that is temporarily prevalent and widespread. The highest incidence is during late pregnancy.
It consists of the displacement of some organ or tissue causing eversion of a part, or the whole, of the vaginal wall so that the membrane is visible between the lips of the vulva. The most common prolapses to occur in sheep involve the eversion of the whole vagina in which the reproductive organs are displaced.
Factors influencing vaginal prolapse:
This study gathered data from 201 farms across Southland and Hawkes Bay for the 2000 and 2001 lambing years. The average prolapse incidence for each of these regions across each year is shown in graph 1 below.
The average incidence of the prolapse on Southland and Hawkes Bay farms ranged from 0.7 - 2.0 % of mixed-age (MA) ewes. The true range of incidence of prolapse across all the study farms ranged from 0 to 5.9%. Other studies have reported an incidence as high as 12%.
The graph below shows the relative risk of vaginal prolapse for MA ewes carrying either twins or triplets in comparison to MA ewes carrying singles. Relative risk ratios are used to show the chance of something occurring more, less or the same as another parameter.
The risk that a single-bearing ewe has a prolapse is 1. If the risk of a twin-bearing ewe having a prolapse is greater than 1, it means that it has a greater risk of prolapse when compared to the single-bearing ewes. If the risk is >1, then there is a lower risk of prolapse.
The risk of a bearing occurring in a mixed-age ewe in Southland was 3.79 times greater for ewes carrying twins and 8.48 times greater for ewes carrying triplets in the 2000 lambing year. The greatest risk was for triplets-bearing ewes in Hawkes Bay in 2001, with a 19.47 times greater chance of having a prolapse. This is a much higher risk than the single-bearing ewes!!
Note that ewes carrying singles are set as a relative risk of 1 for each year and for each region. This means that you cannot compare these risks across years and regions.
Overall, the risk of a bearing occurring in a mixed-age ewe was 5.31 times higher for ewes carrying twins and 11.3 times higher for ewes carrying triplets, compared with single-bearing ewes. This means that in general, ewes carrying triplets had a greater risk for bearings that other ewes.
The graph below shows the percentage of prolapses in ewes that were shorn around 90 days prior to mating or were not shorn. There was a slightly lower incidence of prolapses in the ewes that were shorn prior to mating.
The risk factors that were considered in this study included:
Body condition score
Access to salt in late pregnancy
Terrain in lambing paddocks
Weight gain from mating to scanning
Swedes fed in later pregnancy
Multiple lambs at scanning
Shorn within 72 days prior to lambing
They were tested with an estimate of odds ratio from a mixed effects regression model. This means that the base option or reference category was 1.0 (e.g. no access to salt) and the other option (access to salt) was a multiplier e.g. 1.40. This multiplier means that ewes that had access to salt in late pregnancy had a 1.40 times increased risk of prolapse compared with ewes that did not have access to salt at the base risk of 1.0.
There was no association between incidence of prolapse and body condition score (BCS) at mating, scanning or set stocking prior to lambing. There was also no association between incidence of prolapse and ewe tail length.
The moderate to steep terrain lambing paddocks had 1.37 times increased risk compared to the lambing paddocks with a mixture of flat and rolling topography. Many farmers already prioritise flat paddocks for lambing in general for the ease of access and to reduce the risk of mis-mothering, not necessarily to reduce the risk of prolapse.
If ewes gained weight between mating and scanning they had a 1.29 higher chance of prolapse than ewes that maintained or lost weight. The authors suggested the sparing effect of weight loss in this association may be explained by a nutrition-related negative effect on placental development and the placenta's subsequent effect on the size of the lambs at birth. This was backed up by a paper that under-fed ewes and studied the early placental development.
If swedes were fed in late pregnancy then the ewes were 1.45 times more risk of prolapse. The feeding of swedes and access to salt (mentioned before) were newly identified associations. Both swede and salt intake also result in high water intakes perhaps resulting in ewes having full bladders more often. There has been a belief that swollen bladders from sitting for long periods of time resulted in excessive straining when urinating that promotes prolapse. However, the authors recognised that these results should not be taken as proof of causality and it is likely that a research study would be required to evaluate this hypothesis.
Ewes that were shorn in mid-pregnancy (72 days prior to lambing) then the risk reduced by a quarter that of the ewes that were not shorn. This was not surprising due to the reduced risk also associated with shearing prior to mating. This shows that the timing of shearing didn't matter, both would have a protective effect on vaginal prolapse.
Overall, prolapses are relatively rare ranging from 0 to 5.9% depending on the farm and in general, changing on farm practices to reduce this by 0.1% may not be practical. However, the author suggested that if prolapses are an ongoing problem then management practices in several areas may be warranted.
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Nutrition on bearing incidence
The next study looks at the effect of treating sheep during pregnancy with a Vitamin A, D, E formulation consisting of Vitamin A, D and E on the incidence of vaginal prolapse. Vitamin D was investigated as it had been suggested Vitamin D deficiency may be associated with pelvic floor dysfunction and urinary in humans.
Pregnant ewes from a single farm were separated into 3 treatment groups:
Early Vitamin - vitamin A, D, E injection early and late pregnancy (n=535)
Late Vitamin - vitamin A, D, E injection late pregnancy (n=717)
Control - no vitamin injection (n=764)
The farm was a hill country sheep and beef farm near Waipara in the Canterbury region. All ewes were composites consisting of 75% Coopworth and 25% Texel.
The graph below shows the difference in prolapse incidence between the three treatment groups. The injectable vitamins A, D and E to pregnant ewes reduced the incidence of vaginal prolapse during the period from pregnancy scanning to set-stocking. The control group had an incidence of 5.3% whereas the two treatment groups, early vitamin and late vitamin had similar incidences of 1.8% and 1.4%, respectively.
Similar to the study above, ewe carrying triplets had a greater incidence of prolapse (6.6%) compared with ewes carrying twins (2.8%) or singles (1.8%).
The authors concluded that this study had a restricted data collection period due to only recording sheep records from mating to lambing across 1 year. This study should be replicated and measures made across the complete lambing period, across a number of years. More years data is valuable as there could be a season effect, as shown in the first study, that cannot be identified in data collected in 1 year only.
This has been done during the 2019 season and the results are currently being written up. Stay tuned for a future post when these results are published.
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Find the full text of the article here:
Allott B, Dittmer K, Kenyon A, Elder P. 2020. Preliminary investigation of the effect of treating sheep during pregnancy with a vitamin A, D, E formulation on the incidence of vaginal prolapse. New Zealand Veterinary Journal 68: 193-197.