Masters Theses

Date of Award

12-1977

Degree Type

Thesis

Degree Name

Master of Science

Major

Animal Science

Major Professor

James K. Miller

Committee Members

C.C. Chamberlain, E.W. Swanson

Abstract

Earlier work has indicated dietary iodine requirements of cattle may be increased by some of the protein supplements commonly used in cattle rations. This was investigated further in three experiments in which dairy-beef crossbred calves were fed different amounts of soybean or cottonseed meal. In the first experiment five calves initially weighing 144 kg were fed hay and noniodized salt, alone or supplemented daily with 2.7 kg each of soybean (SBM) or cottonseed (GSM) meal. All calves were fed grass clover mixed hay without protein supplements for 4 weeks (Period 1), During the next 11 weeks (Period 2), in addition to hay, SBM was fed to three calves and GSM to two calves. During the next 10 weeks, (Period 3) when no supplemental protein was fed, 8 mg potassium iodide was given each calf every other day in weeks 7 and 8 (Period 3a) and discontinued in weeks 9 and 10. Protein supplements fed during Period 2 were then reversed for 11 weeks (Period 4). Thyroid function, as measured by plasma protein-bound iodine (PBI), plasma thyroxine (T₄) and thyroid secretion rate (TSR) in the last 2 weeks of each period, was markedly reduced in Periods 2 and 4 as compared to Periods 1 and 3. Results were similar whether SBM or GSM was fed. Fecal excretions of ¹²⁵I (given orally as NaT) and ¹³¹I (from intravenous thyroxine) were not changed by supplemental protein but urinary loss was moderately decreased suggesting an effort to conserve iodine. Nine calves with an initial average weight of 136 kg were assigned to three replicates of a Latin square design in the second experiment. This experiment differed from experiment 1 in that rations were kept isonitrogenous by comparison of SBM and CSM with a low residue isolated soy protein and protein sources were fed at lower levels more comparable to practical rations. No treatment differences in FBI, T₄, or TSR were detected, indicating either that SBM and CSM at these levels had no measurable effect or isolated soy protein was equally as effective. In the third experiment, nine 200-300 kg calves were divided into three groups fed forage alone or supplemented with SBM or CSM. These rations were continued for at least 5 weeks before calves were sacrificed. Digestive tracts were divided into rumen, omasum, abomasum, six equal lengths of small intestine, cecum, and two equal lengths of large intestine. Assays of butanol extracts of digests indicated twice as much T₄ in small intestinal contents of calves fed either SBM of CSM than in those fed forage only. These three experiments considered together indicated SBM and CSM fed at high levels may reduce absorption of thyroxine from the digestive tract and reduce the thyroid function measurements FBI, T₄ and TSR. However, these natural feed ingredients did not have sufficient iodine binding capacity to substantially increase iodine loss as measured by radioactive iodine.

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