Serum total iodine concentrations in pasture-fed pregnant ewes and newborn lambs challenged by iodine supplementation and goitrogenic kale1,2

Abstract

Iodine deficiency can impair the reproductive performance of livestock and affect perinatal mortality of offspring, yet diagnosis of deficiency is complicated and guidelines for I supplementation are imprecise. We challenged pasture-grazing pregnant ewes with a long-acting I supplement and a goitrogenic forage, then monitored their I status during gestation and lactation and in their lambs from birth to weaning. Approximately 46 d into gestation, 376 ewes were assigned to 6 groups comprising 3 supplementation levels × 2 diet regimens. On d 0 the groups received an intramuscular injection of iodized oil providing 0, 300, or 400 mg of I. They grazed until d 23, then half of each supplementation group were fed brassica kale until d 85, then all groups returned to pasture for lambing (parturition approximately d 99) and remained there until weaning (d 192). Serum total I concentration (STIC) was measured repeatedly in 8 'monitor' ewes per group and in their lambs and in milk sampled postpartum. Severity of goiter was determined as the thyroid-weight:birth-weight (TW:BW) ratio in 82 newborn dead lambs. Mean ± SE STIC for all ewes was initially 42 ± 2 (range 24 to 105) µg/L. Diet did not affect I concentrations in ewe serum or milk. Responses to iodized oil were proportional to dose level; STIC increased to approximately 150 and 240 µg/L for the 300- and 400-mg I groups and remained greater than 0-mg I groups for 161 d (P < 0.05). Milk contained 26, 271, and 425 µg I/L for the 0-, 300-, and 400-mg I groups, respectively. Mean STIC of lambs from supplemented ewes did not differ by diet; concentrations for the 300- and 400-mg I groups were 237 and 287 µg I/L at birth, and by weaning all groups were similar (62 ± 3 µg/L). Lamb STIC measured at birth correlated with exposure to I in utero (R(2) = 0.59), which was estimated from the area under the curve (AUC) of ewe STIC measured during the last 99 d of gestation. Thyroid enlargement in lambs affecting the TW:BW ratio was a sensitive indicator of maternal nutrition, being greater with kale feeding (1.27 vs. 0.51 g/kg) and lesser with I supplementation (0.35 vs. 1.44 g/kg). Results support the use of STIC as a biochemical criterion. It was sensitive to the effects of I supplementation with responses in ewes and lambs proportional to dose level and it reflected the relationship between ewe and lamb I metabolism. However STIC did not discriminate between groups of ewes fed pasture vs. goitrogenic forage during pregnancy.

Invited review: impact of specific nutrient interventions during mid-to-late gestation on physiological traits important for survival of multiple-born lambs

“…Lamb birth weight is negatively affected by grazing ewes on kale crops during gestation, an effect which Nutrients for lamb survival is reversed with maternal iodine supplementation during pregnancy (i.m. : iodised arachis oil; high = 400 mg, medium = 300 mg; Knowles and Grace, 2015). Kale is a complementary forage crop that has low iodine concentrations and contains glucosinolates that inhibit thyroid utilisation of iodine through releasing thiocyanate goitrogens (Stoewsand, 1995), therefore in this case, maternal iodine supplementation was correcting for iodine deficiency.…”

Effect of Iodine Supplementation on Physiological Responses and Metabolic Rate of Saidi Pregnant Ewes and the Performance of their Lambs

“…Iodine deficiency should be included in the list of differential diagnoses for poor reproductive performance in ewes and the investigation of high perinatal lamb mortality rates (Sargison et al, 1998). Iodine deficiency affects cellular energy, growth, and differentiation, and the deficiency "among ruminants" manifests primarily as diminished reproductive performance in dams and increased perinatal mortality of offspring (Campbell et al, 2012;Knowles and Grace, 2015). The absolute iodine requirement is difficult to determine because of adaptive responses to varying iodine intake (Steve and Bill, 2012).…”