Selenium metabolism in the dairy cow: the influence of the liver and the effect of the form of Se salt

The effect of supplementary selenium source on apparent and true absorption, retention, performance, and selenium status in lactating Holstein cows

Selenium is an essential trace mineral for dairy cattle and can be provided in the diet in various forms that may differ in bioavailability. The objective of this study was to determine how source of Se affects animal performance, Se status, retention, and apparent and true absorption. Multiparous Holstein cows (n = 24; 597 ± 49 kg of BW) were blocked by DIM (161 ± 18) and randomly assigned to receive 0.3 mg Se/kg of DM (100% of National Academies of Sciences, Engineering, and Medicine requirements) of either organic Se (ORG; selenized yeast) or inorganic Se (INO; sodium selenite). The Se premix was top dressed on a common TMR fed daily and mixed into the top 15 cm directly before feeding. Following an 11-wk adaptation period, cows received simultaneous infusions of an intraruminal isotope dose of 77Se in the same chemical form as the premix, and an intravenous dose of 82Se in an inorganic form. Infusions were followed by a 4-d period of blood and rumen fluid sampling, and total collection of feces, urine, and milk. Daily DMI (23 ± 0.6 kg), milk yield (35 ± 1.2 kg), and serum Se (0.11 ± 0.003 µg/g) were not different between treatments during the adaptation period, but milk Se concentrations were greater for ORG compared with INO. Serum 77Se maximum concentration and area under the curve (AUC) were not different between treatments for 72 h following infusion, but rumen fluid 77Se AUC was higher for ORG than INO. Apparent absorption (64% ± 1.4%), and retention (44% ± 1.5%) of the 77Se dose did not differ between treatments. True absorption was calculated using 82Se enrichment in serum and feces and was determined to be 69% ± 1.3% and did not differ between treatments. Fecal excretion of the 77Se dose was not different between treatments (36% ± 1.4%), but ORG had lower urinary excretion and higher milk excretion compared with INO. These results indicate that organic Se resulted in greater Se concentration of milk and lower urinary Se excretion into the environment, but absorption, Se status, and performance of the cow were not affected by Se source at this supplementation level.

Supplementation of organic and inorganic selenium to late gestation and early lactation beef cows effect on cow and preweaning calf performance

Angus × Simmental cows ( = 48; BW = 595 ± 17.4 kg, BCS = 5.26 ± 0.05, and age = 2.3 ± 0.07 yr), pregnant with male fetuses, were used to determine the effect of Se source during the last 80 d of gestation and first 108 d of lactation on cow and calf performance. At 203 d in gestation, cows were blocked by BW, breed composition, age, and calf sire and randomly allotted to organic Se, inorganic Se, or no Se treatments. Diets contained corn silage, corn stover, haylage, dried distillers' grains with solubles, and minerals and were formulated to contain 10.4% CP and 0.90 Mcal/kg NEg during gestation and 12.1% CP and 1.01 Mcal/kg NEg during lactation. Diets were fed daily as a total mixed ration and none, 3 mg/d Se as sodium selenite, or 3 mg/d Se as Sel-Plex were top-dressed daily. At 68 d postpartum (DPP), milk production was calculated using the weigh-suckle-weigh procedure and a milk sample was collected to determine composition. At 108 DPP, cow-calf pairs were commingled until weaning at 210 DPP. Cow BW and BCS ( ≥ 0.56) did not differ between treatments at any time point during the study. Milk production, milk fat, and total solids ( ≥ 0.38) did not differ among treatments. Milk protein tended to increase in cows fed inorganic Se compared with cows fed organic Se ( = 0.07) and milk lactose tended to be greatest in cows fed organic Se ( = 0.10). Conception to AI and overall pregnancy rates did not differ between treatments ( ≥ 0.39). Calf weights and ADG did not differ through 108 DPP ( ≥ 0.77) or for the preweaning period ( ≥ 0.33). Plasma Se concentration was adequate for all cows and did not differ among treatments for cows ( ≥ 0.37) or calves ( ≥ 0.90). Liver Se concentrations in cows fed inorganic or organic Se were greater than in control cows ( < 0.01). Longissimus muscles biopsies taken from progeny at 108 DPP also did not differ between treatments ( = 0.45). In conclusion, dietary Se source did not affect cow performance, milk production, or reproductive efficiency. Organic Se decreased milk protein and increased milk lactose but did not alter preweaning performance of progeny from Se-adequate cows.

Selenium toxicokinetics after oral and intravenous administration in buffalo calves

The blood levels, toxicokinetics and urinary excretion of selenium were investigated in healthy male buffalo calves after single oral and intravenous administration of selenourea at the dose rate of 0.75mg/kg (providing 0.48mg/kg selenium). The concentration of selenium in blood and urine was estimated spectrophotometrically. Following administration of the drug, the blood selenium disposition patterns exhibited two distinct peaks. The toxicokinetic parameters of selenium were determined by employing non-compartmental analysis. The values of AUC, t(1/2elm), Cl(B) and Vd(SS) were 18.46μgml(-1)h, 10.33h, 20.04mlkg(-1)h(-1)and 0.3lkg(-1), respectively, after oral administration and 23.97μgml(-1)h, 7.12h, 20.53mlkg(-1)h(-1) and 0.2lkg(-1), respectively, following intravenous injection of selenourea. The value of MRT was higher after oral dosing. The bioavailability of selenium, following oral administration of selenourea was 77%. Approximately, 22% of the total intravenous dose and 5.9% of total oral dose of selenium was excreted in urine within 24h of administration of selenourea. The data on blood Se levels may be of help in diagnosing the impeding selenium toxicosis and thus preventing mortality due to selenium toxicity.

A review of selenium deficiency in grazing ruminants Part 1: New roles for selenium in ruminant metabolism

SELENIUM AVAILABILITY: Selenium deficiency has been an important source of loss to the pastoral industries of New Zealand. About 30% of farmed land in New Zealand is considered to be selenium-deficient and continued development of soils and pasture will tend to further decrease the concentration of selenium in pasture.

BIOLOGICAL FUNCTIONS

Formerly it was believed that all biological functions of selenium in animals could be attributed to the antioxidant activity of the enzyme glutathione peroxidase. More recently, it has been shown that selenoproteins have roles in immune function and thyroid hormone metabolism.

RESPONSES TO SUPPLEMENTATION

Following supplementation of ruminants grazing pastures deficient in selenium, milk production and growth responses are likely to occur in cattle, while in sheep improvements in growth and fertility are most likely. Reproductive dysfunction may not be as important as previously thought in cattle grazing pasture moderately deficient in selenium. The relationship between selenium intake and disease resistance deserves further study.

CLINICAL RELEVANCE

Management of selenium deficiency will continue to be important in grazing ruminants. Veterinarians should be aware that many selenoenzymes exist, some with functions quite distinct from the antioxidant role of glutathione peroxidase.

A compartmental model depicting short-term kinetic changes in selenium metabolism in ewes fed hay containing normal or inadequate levels of selenium

Changes in Se metabolism were studied in ewes fed hay containing normal or inadequate levels of Se. After intravenous injection of 75Se-sodium selenite, blood, feces and urine were collected at different times, and the concentrations of labeled and unlabeled Se were determined. Ewes were killed 1, 5, 9 or 14 d after tracer injection, and tissues were obtained for determination of radioactivity and Se concentration. The data were fitted to a compartmental model using the SAAM/CONSAM computer program, and kinetic parameters and steady-state transport rates were estimated. Daily Se intake (Vi) and fecal excretion (VF) were significantly (P < 0.001) higher in the ewes fed normal hay (6.06 +/- 1.09 and 3.36 +/- 0.88 mumol/d, respectively) than in those fed Se-deficient hay (0.64 +/- 0.18 and 0.26 +/- 0.15 mumol/d). The net absorption (Va) of Se was significantly higher in ewes fed normal hay [3.19 +/- 0.82 mumol/d by the balance method, Va = Vi-(VF -Vf) (Vf = endogenous fecal Se) and 1.05 +/- 0.38 mumol/d by using the model (plasma entry rate, U(1))] than in those fed hay deficient in Se [0.57 +/- 0.33 mumol/d (balance) and 0.28 +/- 0.08 mumol/d (model)]. The efficiency of absorption [alpha = U(1) divided by Vi] was significantly higher (0.46 +/- 0.19) in ewes fed Se-deficient hay than in those fed normal hay (0.18 +/- 0.09). Simultaneous fitting of the tracer data of both the groups showed that changes in hepatic extraction and urinary and fecal excretion were sufficient and necessary to account for the kinetic differences observed between treatments.

Metabolic differences and similarities of selenium in blood and brain of the rat following the administration of different selenium compounds

A common intermediate, i.e., selenite, was found in the serum of the rat; the maximum levels occurred 3 h after administration independent of chemical forms. This indicates that both the reduction of selenate to selenite, and oxidation of seleno-dl-methionine to selenite existed in the metabolic pathways of the rat. We found that water-soluble selenium compounds led to a similar maximum content in blood and serum, but seleno-dl-methionine had a higher affinity for the brain and, by gel filtration chromatography, for the higher mol-wt (25-100 K Da) fractions of serum protein, when compared with inorganic forms.

Trace element deficiencies in cattle

Deficiency of cobalt, copper, iron, iodine, manganese, selenium, or zinc can cause a reduction in production. Reduced production occurs most commonly when a deficiency corresponds to the phases of growth, reproduction, or lactation. Because of environmental, nutrient, disease, genetic, and drug interactions, deficiencies of single or multiple elements can occur even when the levels recommended by the National Research Council for these nutrients are being fed. Additionally, random supplementation of trace elements above National Research Council recommendations is not justified because of the negative interaction among nutrients and potential toxicosis. Evaluation of trace element status can be difficult because many disease states will alter blood analytes used to evaluate nutrient adequacy. Proper dietary and animal evaluation, as well as response to supplementation, are necessary before diagnosing a trace element deficiency.

75Selenium-labeled sheep plasma: the time course of changes in 75selenium distribution

Sheep fed rations containing 0.1 ppm selenium were labeled by intravenous injection of radioactive sodium selenite or selenocystine. Gel filtration of serially collected plasma samples indicated that, with time, there was a transition from mercaptan sensitive to high mol wt mercaptan and protein solubilizer resistant selenoproteins. Radiolabeled plasma samples collected from selenite and selenocystine labeled sheep were dialyzed against buffer containing 2-mercaptoethanol or protein solubilizer. No difference in the stability between selenite- and selenocystine-labeled plasma could be detected.

Decreased serum selenium in alcoholics--a consequence of liver dysfunction

The serum concentration of selenium was decreased by 17 and 48% in non-cirrhotic and cirrhotic alcoholics, respectively, as compared to healthy controls. In these alcoholics the serum selenium correlated positively with the serum albumin and plasma prothrombin time and inversely with the serum bilirubin, alkaline phosphatase and gamma-glutamyl transpeptidase. Abstinence from ethanol for two weeks was without effect on the serum selenium level in non-cirrhotic alcoholics and acute alcohol intake did not change the serum selenium concentration in non-alcoholic volunteers. In patients with primary biliary cirrhosis the serum concentration of selenium was similar to that in the alcoholic cirrhotics. In patients with hypoalbuminaemia of renal origin the serum selenium was normal. In conclusion our results show that the deterioration of liver function, irrespective of its aetiology, leads to the decrease in serum selenium levels. Whether a defect in removal of lipoperoxides is associated with this decrease in serum selenium concentration remains to be decided by further studies.

The excretion of selenium in bile and urine of steers: the influence of form and amount of Se salt

1. The excretion of 75Se and stable Se in bile and urine was measured in four steers during 6 h after intravenous injections of 75Se as either selenite or selenate containing either 5 or 5000 microgram carrier Se. 2. When 5000 microgram Se were given, the rate of urinary excretion and plasma clearance of 75SE was similar for both salts. Approximately 23% was excreted in urine and plasma clearance was triexponential, the mean half-life (t 1/2) of the successive components, alpha, beta, and gamma, being 2.3, 15-2 and 465 min respectively. The amount of 75Se excreted in bile was small; 1.94% of the 75SeO3(2-) and 0.86% of the 75SeO4(2-) dose. 3. When 5 microgram Se were given the plasma clearance of 75Se was initially biexponential but the entry of 75Se-labelled protein from the liver caused an increase in plasma radioactivity after 30-40 min. The effect was most marked after 5 microgram 75SeO3(2-) when plasma 75Se radioactivity returned to 69% of the activity present at 2 min. Values for t 1/2 of the two components of clearance for 75SEO4(2-) were respectively alpha 2.6 and 2.5 min, and beta 15.9 and 36.6 min. Similar amounts of 75Se appeared in bile (0.2% of the dose) after injection of either salt but much less 75Se was excreted in urine after 75SeO3(2-) (6%) than after 75SeO4(2-) (17%). 4. At low dosage rates (5 microgram) Se is more readily incorporated into tissues from SeO3(2-) than from SeO4(2-).