Iodine content in egg yolk during its excessive intake by laying hens

Table eggs purposefully enriched with iodine on the basis of its luxury int ake by laying hens can occasionally supplement the human need of iodine. Th e objective of the present paper was to define the level of iodine load of layers compatible with the animal health and with the aim to achieve the re quired iodine content in egg yolk. Iodine load was studied in four groups o f layers of the type Brown Hisex. All hens received complete feed mixture f or layers N-1 nd libitum that contained 0.3-0.7 g I/kg of mixture N-1. In t he first experimental stage, groups B, C, D received complete feed mixture N-1 enriched with other iodine supplements containing 1.3-20 mg/kg of mixtu re N-1 for 129 days within the time schedule shown in Table I. Iodine in fo rm of potassium iodide was applied to mixture N-1. The response to iodine l oad of layers was evaluated by comparing iodine contents in the egg yolk of control group A and of experimental groups B, C and D (Figs. 2, 3 and 4). Group B received another 24-day load of 20 mg I per I kg of mixture N-1 in the second experimental stage following 60-day interruption of additive iod ine application. To determine iodine content in egg yolk a colorimetric met hod according to Sandell-Kolthoff (1937) as modified by Bednar er al. (1964 ) was used after alkaline digestion. The range of average iodine content in egg yolk was recorded in group A - control without any iodine addition to mixture N-1 - at the time intervals shown in Table II (from 870 +/- 217 to 4 767 +/- 234 mu g/kg fresh yolk). An increase in iodine content in the bas ic diet from 0.3 to 0.7 mg/kg of mixture N-1 between 28th and 95th day of e xperiment was accompanied by an increase in iodine content in egg yolk in g roup A (Fig. 1). Iodine content in egg yolk of groups B, C and D was signif icantly higher (P < 0.01) than in group A at the separate stages of iodine additive application (Table II). The maximum value 43 076 +/- 5 738 mu g/kg yolk was calculated as a result of addition of 15 mg I per kg mixture N-1 (Fig. 4). The rate and the curve of iodine content increase in egg yolk are related to the preceding iodine load (Figs. 2, 3, 4, 5). A significant inc rease in iodine content in egg yolk was determined in layers without any pr eceding load or with a load below 7 mg I per 1 kg of mixture in the first w eek after iodine additive application. Another stepwise increase continued to maximum values between 3rd and 10th week of luxury iodine intake. This s tage was followed by a decrease in iodine content in egg yolk to the level that significantly exceeded the values of control group A (Table II). No fu rther increase in iodine content in egg yolk was observed after addition of 20 mg I per 1 kg of mixture N-1 in layers with a preceding load of 15 mg I per 1 kg mixture N-1 (Fig. 4). A delayed increase in iodine content in egg yolk was recorded in 11 and 24 days after iodine application in the second experimental stage in group B with a load of 20 mg I per 1 kg mixture N-1 (Fig. 5) following the two-month interruption of iodine additive applicatio n. An addition of 3.5 mg I per 1 kg of mixture N-1 (Fig. 2) to diets for la yers without any previous iodine load was found best for the production of iodine-enriched eggs as an occasional supplement to cover the iodine need o f humans. Following a 10-week load of layers it is possible to produce eggs with an average content of 5 000 mu g I per 1 kg fresh yolk in the first t hree weeks and of 17 000-20 000 mu g I in the next seven weeks.