Iodine deficiency in sheep is an issue in New Zealand. It affects lambing percentages and lamb survival before birth. Lambs that have iodine deficiency have a low metabolic rate, have a high risk of hypothermia and have difficulty suckling.
Grazing of brassicas during late pregnancy can increase the incidence of goitre (swelling in the neck resulting from an enlarged thyroid gland) in lambs because the crops tend to be low in iodine while also releasing a compound that interferes with the uptake of iodine by the sheep (goitrogenic isothiocyanates).
There are potentially many cases of marginal iodine deficiency that go unnoticed and it is likely these flocks would benefit from iodine supplementation.
In this summary we take a look at a paper that has tried to find a practical approach to preventing iodine deficiency in sheep.
Key Points
If feeding brassica crops during pregnancy then supplement ewes with iodine.
The most reliable way to diagnose iodine deficiency is post-mortem examination of the thyroid gland in new born lambs.
If any lambs' thyroid-weight:birthweight ratios are higher than 0.8 g/kg then supplement the ewes with iodine.
In this study two case study farms were used, one in Southland and one in the Manawatu. Sheep were exclusively fed pasture, or pasture plus brassica fed during late gestation.
The Manawatu farm was located near Bulls and was completed in 2004.
There were 4 treatment groups with 89 sheep in each:
unsupplemented and pasture
iodine supplemented and pasture
unsupplemented and fed pasture and brassicas (kale)
iodine supplemented and fed pasture and brassicas (kale)
The Southland trial was located near Gore and was completed in 1999.
The were four treatment groups with 350 sheep in each:
unsupplemented and fed pasture
iodine supplemented and fed pasture
unsupplemented and fed pasture and brassicas (swedes)
iodine supplemented and fed pasture and brassicas (swedes)
The iodine supplement used in the Manawatu trial was SMARTShot Selenium plus Iodine in the form of iodised oil and the Southland trial used Flexidine, a commercial injectable iodised oil supplement.
The supplement was administered once before the mating period.
Blood samples were collected at docking to determine iodine concentration in serum. Dead lambs were examined to determine the body weight and the thyroid weight.
The graph below show the serum iodine levels of ewes after giving birth in both the Manawatu and Southland trials.
The mean serum iodine concentrations were not different among the unsupplemented groups at docking.
The unsupplemented pasture fed were not different from the unsupplemented brassica fed ewes shown by the two blue bars on the left (Manawatu), however the iodine supplemented ewes in the Manawatu groups had significantly higher serum iodine (240-257 ug/L) compared with the unsupplemented groups (52-57 ug/L).
The iodine supplemented ewes in the Southland trial had iodine levels only 50% greater than the unsupplemented ewes (65 vs 43 ug/L and 45 vs 29 ug/L), however were not significantly different due to the error overlapping.
The large difference between these two locations and the treatment groups within each location, show that the use of serum iodine could be inaccurate for predicting iodine deficiency in a flock. Alternatively, the iodine supplement used in the Southland trial might not been as effective as the Manawatu supplement as these were different products.
The other measure used to determine if the iodine supplementation was beneficial was to weigh the dead lambs and their thyroid to calculate a thyroid to birthweight ratio. An enlarged thyroid compared to body size is a sign of iodine deficiency.
The graph below shows the probability that the flock will respond to iodine supplementation based on the thyroid weight to birthweight ratio of the dead lambs. This was predicted using the data from the unsupplemented and brassica fed treatment groups. This included 154 thyroid weight to birthweight ratios.
This graph shows that the higher the thyroid weight-birthweight, the greater chance that there is iodine deficiency in the flock so would therefore benefit from iodine supplementation. A ratio of 0.80 g/kg was shown to identify with 90% probability that a lamb came from an unsupplemented flock.
This is based on the measurements from individual lambs, so this means a flock with a thyroid weight to birthweight ratio greater than 0.8 g/kg would benefit from iodine supplementation.
The authors recommended measuring the thyroid weight to birth weight ratios in 5-15 lambs and evaluating on an individual basis instead of a flock mean value.
Serum iodine values can still be a valuable measure of iodine status in the flock, however, there is not a defined line where iodine supplementation would be recommended. The most reliable way to diagnose iodine deficiency is post-mortem examination of the thyroid gland in new born lambs.
The authors concluded with the following recommendations.
If feeding brassica crops, then supplement ewes with iodine.
If any lamb thyroid-weight:birthweight ratio is greater than 0.8 g/kg, then supplement ewes with iodine.
If all or most thyroid-weight:birthweight ratios are less than 0.4 g/kg, there is probably no need to supplement ewes with iodine, as the chance of a benefit is low.
If many thyroid-weight:birthweight ratios fall between 0.4–0.8 g/kg, then the iodine status of the flock is unclear. Supplement the ewes if other evidence points towards risk of iodine deficiency, such as occurrence of iodine deficiency in the district.
The iodine can be supplemented three to four weeks before the rams go out. Give ewes and ewe lambs a single injection of a long-acting iodised oil such as Flexidine (subcutaneously as 400mg iodine/ewe).
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