The effect of liveweight and body condition score on the ability of ewe lambs to successfully rear their offspring

Highlights

• Ewe lambs that were heavier were more likely to successfully rear a lamb(s) to docking.

• Ewe lambs that gained greater liveweight during pregnancy were more likely to successfully rear a lamb(s).

• Ewe lambs that were of greater body condition were more likely to successfully rear a lamb(s).

• Commercial farmers can identify ewe lambs that are at risk of being dry at docking.

Abstract

Ewe lamb breeding is a means for farmers to further increase the number of lambs available for sale each year while concurrently increasing the ewe lamb’s lifetime productivity. This study included 7666 replacement ewe lambs from two commercial New Zealand sheep farms that were presented for breeding during their first breeding season (aged 7–8 months) and were subsequently identified as pregnant. Ewe lambs were weighed and body condition scored (BCS) immediately pre-breeding, at pregnancy diagnosis (PD) and immediately prior to lambing (set-stocking). Logistic regression models were developed to assess the effect of liveweight and liveweight changes (both conceptus adjusted and non-adjusted) and BCS at breeding, PD and set-stocking on the risk of failure to rear a lamb (dry) to tail removal and castration (docking) where lambs are three to six weeks of age. There was no effect (p > 0.05) of breeding weight on the risk of being dry. There was an effect (p < 0.001) of conceptus adjusted liveweight at PD, and at set-stocking, such that ewe lambs with heavier conceptus adjusted liveweights were less likely to be dry. There was also an effect (p < 0.001) of weight change between PD and set-stocking on the risk of being dry, such that the more ewe lambs gained in conceptus adjusted liveweight the less likely there were to be dry. The above relationships were also observed with non- adjusted (actual) liveweights. There was an effect (p < 0.05) of BCS at PD and of BCS at set-stocking, such that ewe lambs that were of greater BCS were less likely to be dry. These findings enable commercial farmers to identify ewe lambs within a flock that are at increased risk of failing to successfully rear a lamb(s) to docking. Farmers are then able to plan management prior to breeding and throughout pregnancy to ensure ewe lamb weight and BCS targets are monitored, met and achieved.

Introduction

Currently in New Zealand the sale of lamb in cross bred flocks is a greater contributor to sheep farm income than wool (Anonymous, 2015). Ewe lamb breeding (7–9 months of age at breeding) is a means to further increase the number of lambs available for sale each year while concurrently increasing the ewe lamb’s lifetime productivity (Corner et al., 2013, Kenyon et al., 2011, Kenyon et al., 2014b). However, less than 40% of New Zealand farmers choose to breed their ewe lambs (Kenyon et al., 2014b) indicating that there must be limiting factors which are restricting the uptake of this management option.

Management practices required to maximise the likelihood of a ewe lamb becoming pregnant are well documented (see review Kenyon et al., 2014b). However, farmer and veterinary evidence suggest losses between pregnancy diagnosis and marking (docking), when lambs are approximately three to six weeks of age, continue to be an issue on commercial New Zealand farms (Kenyon et al., 2014b, Ridler et al., 2015). Lower survival rates of lambs born to ewe lambs compared with those born to mature ewes have been reported, although few studies have directly compared this (Corner et al., 2013, Kenyon et al., 2014b). Corner et al. (2013) reported lamb survival to weaning of 69–89% for offspring born to ewe lambs, compared with 83–96% with mature ewes.

The development of ultrasound pregnancy diagnosis in sheep has enabled farmers to identify and cull their non-pregnant ewe lambs, while palpation of udders at docking enables actively lactating (wet) ewe lambs and not actively lactating (dry) ewe lambs to be identified. Those that are identified as dry at docking are assumed to have lost their lambs(s) between pregnancy diagnosis and docking. The aim of this study was to investigate the effects of liveweight and body condition score (BCS) at breeding, pregnancy diagnosis and at set-stocking on the ability of ewe lambs to rear their offspring to docking, on commercial New Zealand farms.