Lamb birth weight

Tim Keady, Teagasc Researcher defines optimum birth weight and lists the benefits in achieving this. He analyses the relationship between birth weight and lamb mortality and he stresses the importance of having a good nutrition plan in place to achieve optimum birth weight.

Main Messages

• Each 0.5 kg increase in birth weight improves weaning weight by 1.7 kg
• Optimum birth weight is the weight at which lamb mortality is minimised
• Optimum birth weight varies according to litter size
• Implement a nutrition plan  in which (a) concentrate feed is targeted according to litter size (b) concentrate feed level increases as lambing date approaches and (c) silage feed value is determined by laboratory analysis
• A poor nutrition plan will result in high lamb mortality due to weak/light lambs in large litters and large lambs, thus dystocia, particularly in singles

Introduction

Many factors affect lamb birth weight including litter size, gender, genotype and nutritional management of the ewe during late pregnancy. Optimum birth weight is the weight at which lamb mortality is minimised. Achieving the optimum birth weight reduces labour associated with lambing difficulty and very small lambs, reduces lamb mortality and improves flock productivity. Nutritional management during this critical time will have a major impact on lamb growth in utero and thus birth weight. Achieving the optimum birth weight reduces labour associated with difficulty lambing large lambs and caring for very small lambs, reduces lamb mortality and improves flock productivity. My aim in this article is to highlight the importance of birth weight on lamb performance and on mortality.

Effects of birth weight

The birth weight of lambs influences subsequent growth rate and, consequently, weaning weight.  Previous studies at Athenry have shown that for each 0.5 kg increase in lamb birth weight subsequent weaning weight (at 14 weeks) increases by approximately 1.7 kg.  The increased weaning weight is due to a combination of the increase in birth weight per se and increased growth rate from birth to weaning.

Optimum birth weight is the weight at which lamb mortality is minimised and varies according to litter size. Thus birth weight is a major factor influencing lamb viability. Lamb birth weight effects lamb mortality as small and large lambs are more likely to die. Thus achieving the optimum birth weight reduces labour associated with lambing difficulty and very small lambs, reduces lamb mortality and improves flock productivity. Recent studies at Athenry show that 15% of neonatal lamb mortality (lambs that die within 7 days) occurs due to dystocia (lambing difficulty).

The effect of lamb birth weight on lamb mortality is presented in Figure 1. Regardless of litter size, as lamb weight increases mortality declines initially but reaches a plateau at the optimum birth weight, which varies by litter size. Subsequently, as birth weight increases above the optimum, lamb mortality increases again – probably reflecting difficulties immediately prior to and during delivery. The optimum birth weight, based on lamb mortality, for lambs born as singles, twins and triplets is 6.0, 5.6 and 4.7 kg, respectively. Thus the optimum birth weight for lambs born as twins and triplets is 0.93 and 0.78 times that of singles.

Lamb mortality is also influenced by litter size. For lambs born as singles, twins and triplets mean lamb mortality is 6, 7 and 21% respectively. Consequently as flock prolificacy increases lamb mortality will increase. 

Concentrate feeding management

To optimise the use of concentrate ewes should be grouped according to predicted litter size (based on ultrasonic scanning) and expected lambing date (mating date - raddle colour). As lamb weight increases by 70% during the last 6 weeks of pregnancy the demand for nutrients increases substantially. Consequently, supplementation should be stepped up weekly over the period immediately prior to lambing. If ewes are not housed according to lambing date then a proportion of the flock which are lambing early will not receive adequate nutrient intake and their lambs will be light and colostrum supply could be limited. Ewes which will lamb late will receive high concentrate inputs and thus produce heavy lambs at birth which are prone to dystocia and mortality.  Labour demands are increased by the birth of light lambs due to additional requirements to assist to suckle and/or provide colostrum; and by heavy lambs due to the requirement to assist at birth.