The effects of supplementation with cobalt, and method of administration, on ewe reproduction and offspring performance to weaning

•Ewes in grass-based system supplemented with Co from pre joining to late pregnancy.•Cobalt supplementation increased plasma concentrations of Co and vitamin B12.•Supplementation reduced the incidence of ewes classed as marginal/low for Co status.•Ewes on fortnightly Co drench had higher plasma B12 than those with a Co bolus.•Cobalt supplementation had no effect on ewe productivity or progeny performance. The concentration of Co in the herbage of a large proportion of grassland farms in Ireland is lower than that required by sheep. The effects of supplementing ewes with Co, and method of supplementation, on ewe and offspring performance, were evaluated over 2 years. The ewes (359 in year 1, 241 in year 2), representing 3 genotypes, were allocated to one of 3 treatments. The treatments were: Control - no supplementation; Drench – Co supplementation via oral drench (31.5 mg Co as CoSO4•7H2O) administered every 14 days; Bolus – Co supplementation via rumen bolus (effective life of ∼6 months) that only contained Co as CoSO4•7H2O with an expected Co release rate of 0.6 mg/day in year 1 and 1 mg/day in year 2. Drench treatment was administered from 7 weeks pre joining, until 6 weeks prior to expected lambing date (early – to mid - March). One bolus was administered to ewes on the bolus treatment at the start of the study (day 0) each year. The ewes were managed as one flock in a rotational-grazing system until housing in early December each year; they were shorn at housing and offered grass silage ad libitum plus a concentrate supplement during late pregnancy. Ewes and lambs were put to pasture within 3 days of lambing. Supplementation with Co, via drench or bolus, increased the concentration of Co (p< 0.001) and vitamin B12 in ewe plasma at ∼day 50 and ∼day 100 of pregnancy and these effects were generally greater for Drench than Bolus treatments. Supplementary Co had no effect (p > 0.05) on ewe BW or body condition score at joining (day 49), mid-pregnancy, lambing, 5 weeks post lambing or weaning. Likewise, supplementation had no effect (p > 0.05) on litter size, lamb mortality or number of lambs reared per ewe joined, or on lamb BW at birth or weaning. The only significant animal-performance difference detected between the Bolus and Drench treatments concerned the incidence of lambs classified as ‘born dead or died within 24 h of birth’ (3.1% for Drench, 6.5% for Bolus; p < 0.05). It is concluded that, whilst supplementing ewes (high proportion whom (30 to 6