Risk factors associated with lambing traits

Abstract

The objective of this study was to establish the risk factors associated with both lambing difficulty and lamb mortality in the Irish sheep multibreed population. A total of 135 470 lambing events from 42 675 ewes in 839 Irish crossbred and purebred flocks were available. Risk factors associated with producer-scored ewe lambing difficulty score (scale of one (no difficulty) to four (severe difficulty)) were determined using linear mixed models. Risk factors associated with the logit of the probability of lamb mortality at birth (i.e. binary trait) were determined using generalised estimating equations. For each dependent variable, a series of simple regression models were developed as well as a multiple regression model. In the simple regression models, greater lambing difficulty was associated with quadruplet bearing, younger ewes, of terminal breed origin, lambing in February; for example, first parity ewes experienced greater (P < 0.001) lambing difficulty (1.56 ± 0.02) than older ewes. The association between lambing difficulty and all factors persisted in the multiple regression model, and the trend in fixed effects level solutions did not differ from the trend observed in the simple regression models. In the simple regression analyses, a greater odds of lamb mortality was associated with male lambs (1.31 times more likely of death than females), lambs of very light (2 to 3 kg) and very heavy (>7.0 kg) birth weights, quadruplet born lambs and lambs that experienced a more difficult lambing (predicted probability of death for lambs that required severe and veterinary assistance of 0.15 and 0.32, respectively); lambs from dual-purpose breeds and born to younger ewes were also at greater risk of mortality. In the multiple regression model, the association between ewe parity, age at first lambing, year of lambing and lamb mortality no longer persisted. The trend in solutions of the levels of each fixed effect that remained associated with lamb mortality in the multiple regression model, did not differ from the trends observed in the simple regression models although the differential in relative risk between the different lambing difficulty scores was greater in the multiple regression model. Results from this study show that many common flock-and animal-level factors are associated with both lambing difficulty and lamb mortality and management of different risk category groups (e.g. scanned litter sizes, ewe age groups) can be used to appropriately manage the flock at lambing to reduce their incidence.

Impact of birth and rearing type, as well as inaccuracy of recording, on pre-weaning lamb phenotypic and genetic merit for live weight1

“…The main causes for lamb rearing type to differ from the respective birth type are lamb mortality, cross-fostering or artificial rearing. The incidence and causes of lamb mortality vary by litter size and have been well documented ( Sawalha et al, 2007 ; Geenty et al, 2014 ; McHugh et al, 2016 ). Cross-fostering is a common management practice undertaken worldwide (Australia– Alexander et al, 1985 ; New Zealand– Pollard and Littlejohn, 1999 ; US– Snowder and Knight, 1995 ), with reports of over 93% of producers adopting this management tool ( Ward et al, 2011 ).…”

“…Interestingly, however, no difference was detected in either 40-day weight or ADG in twin-reared lambs born as either singles or twins, corroborating previous sheep research that shows that the rearing type can influence the milk yield of the ewe ( Cardellino and Benson, 2002 ) and the suckling behavior of the lamb(s) ( Hess et al, 1974 ). For triplet-born lambs reared as a single, the heaviest triplet lamb was likely to be reared by the natural dam; the lighter littermates were likely to be less vigorous at birth ( Moore et al, 1986 ; Macfarlane et al, 2010 ) and therefore more likely to be cross-fostered or artificially reared, or even die at birth or soon thereafter ( Sawalha et al, 2007 ; Geenty et al, 2014 ; McHugh et al, 2016 ). As heavier triplet lambs tend to be reared as singles or twins, one might therefore expect a bias toward male lambs, but the proportion of males and females triplet-born lambs reared as singles or twins were equal, indicating that producers had no preference toward the sex of triplet lambs destined for cross-fostering.…”

“…Lamb live weight is recorded at birth and again at approximately 40 days of age by Irish producers using weigh-scales. Lambs with a recorded weight at birth between 2 and 9 kg were retained and lamb birth weight was rounded to the nearest half kilogram ( McHugh et al, 2016 ). Based on the editing criteria used in the national genetic evaluations, 40-day weight was defined in the present study as the live weight taken between 20 and 65 d of age; only records of lambs weighing between 12 and 32 kg were retained.…”

“…The appropriate adjustment is necessary because the pre-weaning growth potential of a lamb is affected by whether the lamb was born or reared as a singleton or multiple ( Safari et al, 2007 ; David et al, 2011 ; Notter and Brown, 2015 ), and birth type can account for up to 31% of the phenotypic variance in pre-weaning growth ( Hagger, 1998 ). Nonetheless, lambs born into a multiple litter are at greater risk of mortality ( McHugh et al, 2016 ) and, where mortality may have occurred, a reduced litter size may not actually be recorded by the producers. Similarly, cross-fostering events may not always be recorded thus introducing errors into the recorded rearing type of a given animal.…”

Effect of Gender on Growth and Carcass Traits of Romney Marsh Lambs

“…Jacobson et al (2020) add that, one of the most important factor which affects the EL is a birth weight. According to McHugh et al (2016), lambing difficulties are also affected by the parity when the most threatened females are primiparous ewes. However, in the present study, all ewes were on the second parity.…”