Identification of risk factors for ewe mortality during the pregnancy and lambing period in extensively managed flocks

BACKGROUND

Ewe mortality during pregnancy and lambing is an issue for sheep producers globally, resulting in reduced productivity and profitability, compromised ewe welfare, and poor consumer perception. Despite these negative consequences, there was little investigation into factors associated with ewe death during this time. Therefore, this study aimed to assess associations between ewe body condition score (BCS), weight, reproductive parameters, and risk of mortality during pregnancy and lambing.

METHODS

Four cohorts from three commercial New Zealand farms participated, with 13,142 ewe lambs enrolled and followed over time. Data were collected for five consecutive lambings. Visits aligned with key on-farm management times, specifically: prior to breeding, at pregnancy diagnosis (PD), prior to lambing (set-stocking), and, at weaning of their lambs. At each visit, ewes were weighed, BCS assessed and reproductive status was recorded when relevant (litter size at PD and lactation status after lambing). Ewes that died or were culled were recorded, and any ewes that were absent from consecutive visits were presumed dead. Logistic regressions were developed to assess the relationship between weight and BCS at each visit, PD result (single or multiple-bearing) and lactation status (wet or dry) in each year, and, risk of mortality during the pregnancy and lambing period in each year.

RESULTS

In the PD to weaning period, mortality incidence ranged from 6.3 to 6.9% for two-tooth (18-months-old at breeding) to mixed-age (54-months-old at breeding) ewes. For ewe lambs (7 to 8-months-old at breeding), mortality was 7.3% from set-stocking to weaning. Heavier ewe lambs at PD were less likely to die during lambing (OR: 0.978, p = 0.013), as were those with greater set-stocking BCS. In subsequent years, BCS was a predictor of ewe death, with odds of mortality greatest for ewes < BCS 2.5. Additionally, for poorer BCS ewes, increasing weight reduced risk of mortality, but there was no impact of increasing weight in greater BCS ewes.

CONCLUSIONS

This study identified risk factors associated with ewe mortality during the pregnancy and lambing period. Flock owners can use these to either cull at-risk ewes or proactively intervene to reduce likelihood of mortality, thereby improving flock productivity, profitability and welfare.

Ewe wastage in commercial sheep flocks: A review of current knowledge

Ewe wastage encompasses ewes that do not reach the end of their potential productive lives and is the combination of premature culling and on-farm mortality. Increased ewe wastage results in reduced flock productivity and profitability, primarily driven by the need for increased replacement ewe lambs to maintain flock numbers and the costs associated with these additional replacements (e.g., reduced sale lambs, reduced flock age resulting in lower lamb production, and reduced ability to utilise terminal sires). Annual ewe mortality rates are reported to range from 2.8-40.2% in New Zealand commercial flocks, while wastage over a ewes' productive lifespan (i.e., mortality and premature culling combined to 6-7 years-of-age) is reported to be up to 90%. To date, ewe wastage has received relatively little attention in the published literature.This article reviews published studies describing wastage in non-dairy ewes, focusing on New Zealand commercial flocks, although overseas pasture-based data is also considered. The relevance of, and challenges associated with, investigating ewe wastage are discussed. Ewe culling and mortality data is outlined, and causes are described. Known key risk factors for wastage, namely, reproductive performance, udder health, dental health, body condition score and breeding of ewe lambs, are reviewed. Finally, current knowledge gaps and future areas of research are identified and evaluated.

Ewe Wastage in New Zealand Commercial Flocks: Extent, Timing, Association with Hogget Reproductive Outcomes and BCS

Simple Summary

Ewe wastage is the combination of on-farm mortality and premature culling. Increased wastage results in a reduction in flock productivity and profitability, yet internationally, there is limited research on actual wastage incidence and causes in commercial flocks. This study reports both lifetime wastage and detailed annual wastage for 13,142 ewes from four cohorts on three commercial New Zealand farms. This study also describes the relationship between ewe pre-mating body condition score and wastage. Of the 13,142 enrolled ewes, 50.4% and 40.0% exited their respective flocks due to premature culling and on-farm dead/missing, respectively. Annual mortality incidence ranged from 3.5 to 40.2%. Wastage as a hogget was identified as an area in which improvements can be made to reduce overall wastage. Pre-mating body condition score was a predictor of wastage, with ewes with greater pre-mating body condition score having lower odds of wastage due to both premature culling and mortality. Therefore, farmers should focus on improving the body condition score of their ewes prior to breeding.

Abstract

Ewe wastage is the combination of on-farm mortality and premature culling. Internationally, there is limited research on actual wastage incidence and causes in commercial sheep flocks. To the authors’ knowledge, this is the first study that reports both lifetime wastage and detailed annual wastage in a sample of commercial New Zealand flocks. This study utilized data collected from 13,142 ewes from four cohorts on three commercial New Zealand farms (Farm A 2010-born, Farm A 2011-born, Farm B, Farm C), during the period 2011–2017, as they aged from replacement hoggets to 6-year-old ewes (Farm A and Farm B) or 3-year-old ewes (Farm C). Data collection visits occurred at three or four key management times each year, namely pre-mating, pregnancy diagnosis, pre-lambing and weaning. At each visit, body condition score (BCS) was assessed and any ewes that were culled or had died on farm were recorded. As this was a lifetime study, each ewe was assigned an outcome and corresponding ‘exit age’. By the end of the study, all ewes that had exited their respective flocks, were classified as either prematurely culled, or dead/missing, or if still in the flock, as censored, and either the exact date or interval in which they exited the flock was recorded. Semi-parametric competing risk (premature culling vs. dead/missing), interval-censored survival models were developed to: 1. describe the association between hogget reproductive outcomes and risk of subsequent wastage, and 2. assess pre-mating BCS as a predictor of wastage in that production year. Of the 13,142 enrolled ewes, 50.4% exited their respective flocks due to premature culling and 40.0% due to on-farm dead/missing, giving a total of 90.4% that exited due to wastage. Annual mortality incidence ranged from 3.5 to 40.2%. As a hogget, wastage incidence ranged from 7.6 to 45.4%. Pregnancy or rearing a lamb as a hogget did not increase risk of subsequent wastage. In all years, pre-mating BCS was a predictor of ewe wastage, with odds of wastage lower with increasing BCS. Therefore, farmers should focus on improving pre-mating BCS to 3.5/5.0 by assessing ewe BCS at weaning, allowing poorer-BCS ewes to be managed to gain BCS before re-breeding.

The effects of parasitism on ewes for prime lamb production in western Victoria

Context Internal parasites are estimated to cost the Australian sheep industry AUD436 million per annum (p.a.). Aims To assess the effects of parasitism in flocks producing prime lambs in the 500–700 mm p.a rainfall. area of Victoria. Methods Ewes on two farms that followed ‘best practice’ gastrointestinal parasite control programs (BP) and two farms that did not (regionally typical, TYP) were compared. Separate cohorts of ewes were monitored from pregnancy scanning to their subsequent joining each year for three consecutive seasons. Observations included worm egg count (WEC), bodyweight, condition score and presence of breech soiling (dag). These were compared between groups that were treated to suppress parasitism (SUP) and those treated according to the usual program used on that farm (NSUP). Data from individual ewes were analysed using a multivariable, mixed-effects regression model. Key results After adjusting for known confounders, SUP ewes were 1.2 (95% CI 0.80–1.6) kg heavier than NSUP ewes. Mature SUP ewes were significantly heavier than NSUP ewes at their next joining on 6 of 18 occasions, mostly following winters when ewes experienced nutritional stress. Ewe hoggets and Merino ewes were generally more susceptible to parasitism than mature non-Merino ewes; single-bearing ewes were less susceptible than those bearing twins. The effects of parasitism were reduced when peri-parturient ewes had an optimal condition score and grazed adequate pastures. Conclusions Ewes were more vulnerable to parasitism when immature, twin-bearing, or under nutritional stress. Some of the greatest differences between SUP and NSUP ewes occurred following periods of low feed availability and/or ewe condition score. The difference between the mean bodyweight of SUP and NSUP Merinos was not always greater than that of the non-Merinos. WECs are not a sole reliable indicator of the effects of parasitism in this class of sheep. Implications Immature or twin-bearing ewes, and those in suboptimal body condition, should be managed considering their increased vulnerability to parasitism, and WEC interpreted alongside other factors. Controlled release capsules were not cost effective in reducing production loss from gastrointestinal nematodes in most years but may be effective in reducing the effects of clinical parasitism in some cases.

Seasonally timed treatment programs for Ascaris lumbricoides to increase impact-An investigation using mathematical models

There is clear empirical evidence that environmental conditions can influence Ascaris spp. free-living stage development and host reinfection, but the impact of these differences on human infections, and interventions to control them, is variable. A new model framework reflecting four key stages of the A. lumbricoides life cycle, incorporating the effects of rainfall and temperature, is used to describe the level of infection in the human population alongside the environmental egg dynamics. Using data from South Korea and Nigeria, we conclude that settings with extreme fluctuations in rainfall or temperature could exhibit strong seasonal transmission patterns that may be partially masked by the longevity of A. lumbricoides infections in hosts; we go on to demonstrate how seasonally timed mass drug administration (MDA) could impact the outcomes of control strategies. For the South Korean setting the results predict a comparative decrease of 74.5% in mean worm days (the number of days the average individual spend infected with worms across a 12 month period) between the best and worst MDA timings after four years of annual treatment. The model found no significant seasonal effect on MDA in the Nigerian setting due to a narrower annual temperature range and no rainfall dependence. Our results suggest that seasonal variation in egg survival and maturation could be exploited to maximise the impact of MDA in certain settings.