Perinatal lamb survival was not reduced by grazing forage oats

Introduction

Strategies to reduce the high rates of perinatal lamb mortality (average 30% of twin lambs) in extensive grazing systems are required, which address the key causes, namely dystocia and the starvation–mismothering–exposure complex (Hinch and Brien 2014). The type of pasture grazed may contribute to the risk of ewe and lamb mortality. Cereal forages often contain calcium (Ca), magnesium (Mg) and sodium (Na) concentrations below ewe requirements or imbalances of minerals, and grazing late-pregnant (Masters et al. 2018) or lactating (Herd 1966) ewes on forage such as cereal crops, which are deficient in Ca and Mg, increases the risk of metabolic disease such as hypocalcaemia and hypomagnesaemia; so, supplementation with Ca, Mg and Na for pregnant ewes grazing cereal forages is recommended to reduce the risk (Masters et al. 2019). Subclinical deficiencies of Ca (plasma Ca < 2.0 mmol/L; Suttle 2010) and/or Mg (plasma Mg < 0.74 mmol/L; McCoy et al. 2001) also commonly occur on pastures typical of southern Australia (Hocking Edwards et al. 2018), and potentially contribute to lamb mortality (Friend et al. 2020).

Munn et al. (2024) found that late-pregnant twin-bearing ewes with subclinical deficiency of Ca had lower survival of lambs (76.4% vs 91.2%) to 3 days of age, although survival at birth was not reduced. However, mineral supplementation of subclinically deficient ewes during late pregnancy and lactation did not improve lamb survival (Robertson et al. 2022), and lamb survival has been reduced when grazing forage oats compared with legume pasture when ewes were not deficient in Ca, Mg or Na, suggesting that other factors were causative (Robertson et al. 2024). Given the widespread use of grazing cereals for lambing ewes, further understanding of potential impacts on lamb survival is needed.

There is limited scientific evidence on the impact of grazing cereals compared with other pastures on lamb survival, and the reported impact has been variable (Glover et al. 2008; Oldham et al. 2008; Paganoni et al. 2008). Potential mechanisms for variation in the response, in addition to mineral status, include excessive nutrition during late pregnancy potentially increasing lamb birthweights and the incidence of dystocia (Ocak et al. 2005), fetal number, the duration of grazing and level of nutrition, and degree of shelter provided by pasture height and environmental challenge (Hinch and Brien 2014).

Our previous study (Robertson et al. 2024) indicated a reduction in lamb survival when ewes changed from lucerne-based pasture to forage oats (Avena sativa) at the commencement of the lambing period, but this was associated with a greater loss of ewe condition. The aim of the present study was to further test the hypothesis that grazing a legume-based pasture throughout late pregnancy and the lambing period would improve lamb survival compared with grazing forage oats during the lambing period, when both were supplemented with Ca, Mg and Na.

Materials and methods

Results

Discussion

Lamb survival was not reduced by moving ewes from legume pasture to forage oats for the lambing period. This is in contrast to our previous report (Robertson et al. 2024) where a 13% lower survival was recorded when ewes moved from legume-based pasture to forage oats for the lambing period compared with grazing legume pasture throughout. The contrasting results were associated with loss in ewe condition score during the lambing period. Loss in condition in the periparturient period is associated with higher rates of lamb mortality (Behrendt et al. 2019). Undernutrition during late pregnancy reduces fat reserves in newborn lambs and reduces the quantity of colostrum available after birth (Mellor and Murray 1985), delays lactogenesis (McCance and Alexander 1959) and impairs maternal and lamb behaviours (Dwyer et al. 2003), all of which may adversely affect lamb survival. The contrasting results between our two studies highlight the need to minimise loss in ewe condition over the peri-parturient period, even when ewes have a score >3.0, noting that limits to intake make this difficult to achieve in twin-bearing ewes.

The larger loss in condition during pregnancy for ewes grazing Oats may have adversely affected lamb survival. However, the numerical difference in survival between treatments appeared to be associated with a larger condition loss in lactating ewes grazing Legume. The levels of loss in condition for ewes during the lambing period are consistent with variation reported from commercial properties (Kelly 1992). The level of loss for single-bearing ewes grazing legumes appears high for the level of pasture availability, although it is consistent with the loss in twin-bearing ewes. A higher level of undernutrition for ewes grazing legumes is not indicated by lamb growth rates or marking weights, although utilisation of ewe fat stores would have reduced any decline in milk production (Treacher 1971) and minimised the impact on lamb growth.

Further studies are needed to define the conditions where grazing forage oats may alter lamb survival. The large variation in survival among legume paddocks may have prevented statistical difference between treatments, but is not unusual. Between-paddock variation did not reflect the uniform pasture, topography and shelter within the subdivisions of a single paddock, and was not due to mismothering after tagging lambs. Grazing forage oats rather than legume pasture has some potential to increase lamb survival because of the greater provision of shelter. The taller height of oats would be expected to reduce wind speed for sitting lambs, reducing exposure to chill. A high level of chill causes large increases in lamb mortality (Donnelly 1984), reducing wind speed can produce large reductions in the chill index (Broster et al. 2012) and tall grass windbreaks are an effective method to improve lamb survival (Alexander et al. 1980) in cold, windy weather. However, the weather during the present study was mild, and lambing in forage oats during mild weather has previously been reported to not increase lamb survival (Glover et al. 2008; Paganoni et al. 2008) or only increase the survival of twins (Oldham et al. 2008). It is therefore unclear whether shelter or improved nutrition for ewes grazing oats had the greater influence on lamb survival in the present study. The contrasting literature does indicate that there is no consistency in the effect of grazing forage oats on lamb survival compared with other pastures.

The growth rates of lambs grazing forage oats were not reduced compared with those grazing legume-based pastures, although liveweights at marking were reduced, similar to our previous report (Robertson et al. 2024), and despite higher herbage availability. The reduced marking weight of lambs occurred in association with a lower loss of condition for ewes grazing oats. The lower lamb weight may reflect a lower nutritive value from intake of oats than of legume pasture, because final weights were recorded when the majority of lambs were 28 to 45 days of age. At this age, pasture would have constituted a significant proportion of the lamb’s diet (Hodge 1966). The nutritive value of pastures was not measured, but in August/September, oats was changing from vegetative to reproductive, with a corresponding increase in stem and expected reduction in digestibility and protein content (Kilcher and Troelsen 1973). In addition, the digestibility of oats declines more rapidly with advancing maturity than does that of subclover (Norman and Masters 2023). The protein content of subclover reportedly remains stable at approximately 20% during the vegetative phase prior to wilting and maturity (Stockdale 1992), although higher concentrations have been reported, and are increased with phosphorus fertiliser application (Saul et al. 1999). Although oats forage during the tillering phase contains high protein (>25%), this declines with maturity (Jacobs et al. 2009), such that pre-flowering, the protein concentration of oats has been reported as less than 9%, markedly lower than for annual legumes (Wickham et al. 2007).

In our study, the probable lower protein content of the oats than of legume-based pasture at the time when lambs were consuming forage may have disadvantaged the liveweight of those grazing oats, because of the high protein requirement of suckling lambs (13.1 and 10.9 g crude protein per 1 MJ metabolisable energy at 5 kg and 18 kg respectively) (Danso et al. 2016). In addition, the milk production of ewes consuming lower-protein diets is reduced (Treacher and Caja 2002), which would contribute to lower growth rates of their suckling lambs.

The lower liveweight of lambs grazing oats may have been caused by mineral deficiencies reducing the milk production of ewes, contributing to lower energy demand, and so to reduced loss of ewe condition score. Cereal forage is usually deficient in Na and Ca (Masters et al. 2019), and supplementation of deficient diets with Na has increased the growth rate of grazing lambs and increased the milk production of cows (Joyce and Brunswick 1975). An imbalance of minerals in cereal forage may also contribute to reduced lamb growth rates (Masters and Thompson 2016). However, in the present study, both treatment groups were offered a Ca, Mg and Na supplement, so it appears unlikely that ewes or lambs were deficient in these minerals, given deficiencies were not reported after supplementation under similar circumstances (Robertson et al. 2024).

The target intake of mineral supplement was largely achieved in this study. While mineral concentrations in plasma were not measured due to funding constraints, no symptoms of clinical Ca or Mg deficiency were apparent, indicating that supplementation was effective, given a history of hypomagnesaemia on this property in unsupplemented lactating ewes. It remains unclear whether subclinical deficiencies were present or whether their incidence differed among the treatments, given that supplementation prevented deficiencies in a similar situation (Robertson et al. 2024). If present, they would be expected to occur at a higher incidence in ewes while grazing oats rather than legume pasture (Masters et al. 2018).

The role of subclinical Ca and Mg deficiencies in lamb survival remains unclear. While subclinical Ca deficiency has been associated with a 17.6% lower survival of lambs to 7 days of age, survival to 18 h was not reduced in the study by Munn et al. (2024), which would be expected if the rate of dystocia increased. However, the number of lambs in their study (n ~ 70/treatment) was relatively low. Clinical Ca deficiency increases the incidence of dystocia in cattle and Ca has a role in thermoregulation (Friend et al. 2020). Poor thermoregulation is a consequence of prolonged parturition for lambs with reduced rectal temperatures persisting for 3 days after birth (Darwish and Ashmawy 2011); however, Munn et al. (2024) did not demonstrate a reduction in the rectal temperature of lambs 6–18 h after birth for subclinically deficient ewes, nor in other measures of lamb viability or maternal behaviours, which would indicate prolonged parturition as the cause of the reported reduced survival. Munn et al. (2024) indicated a larger energy deficit associated with subclinically deficient ewes, which through impacts on lactation may have contributed to post-parturient lamb mortality.

In our study the rate of ewe mortality (1.2%) and rate of assisted delivery for ewes, owing to dystocia (0.5%), were low relative to Australian reports for 2–2.5% periparturient mortality (McQuillan et al. 2021) and 4.9% of maiden ewes being assisted (Horton et al. 2018). Our study does not provide any evidence that grazing of forage oats increased the rate of dystocia, and thereby reduced lamb survival, in contrast to anecdotal perceptions (Holst and Killeen 1976; Masters et al. 2018). This may have been due to the short time that ewes grazed oats pre-lambing, such that excessive birthweights did not result. However, our earlier study where ewes grazed forage oats for 6 weeks before the lambing period also did not result in higher birthweights than if ewes grazed legume-based pasture (Robertson et al. 2024). Increased dystocia may also be associated with over-fat ewes (Holst et al. 2002), rather than excessive birthweights, with longer-term nutrition rather than species of forage being the important factor.

Regardless of the cause of reduced liveweight of lambs at marking, the lower weight of lambs grazing oats than of those grazing legume pasture is a disadvantage with potential impacts on weaner survival, feed requirements and weight at sale. The reduced loss of ewe condition over the lambing period for ewes grazing oats is beneficial in reducing the weight gain required prior to the subsequent mating. These impacts may differ if lambing occurred when oats were at an earlier vegetative state, and depending on the relative herbage availability and quality of alternative pasture.

Conclusions

Perinatal lamb survival was not reduced by grazing oats forage rather than legume-based pasture during the lambing period, but the higher number by 0.14 more lambs marked per ewe, although not statistically significant because of low replication, is of practical significance. Grazing oats reduced the loss in condition during the lambing period, associated with abundant green herbage, which may also have provided additional shelter from wind to reduce deaths from hypothermia or camouflage from foxes, both of which have the potential to improve survival. Producers may safely move ewes from legume pasture to graze oats forage during lambing when supplemented with Ca, Mg and Na without adversely affecting ewe or lamb survival, although lamb weight at marking may be reduced. Inconsistent effects on lamb survival in the literature indicate variation in other factors rather than a risk owing to forage oats.