Selenium deficiency is characterised by myopathy and ill thrift in lambs and calves and infertility in ewes and cows (Campbell, 1983). Young selenium deficient animals may die suddenly if the myocardium is affected, show respiratory distress if the intercostals muscles are involved and walk stiffly or reluctantly if affected in the leg muscles (Caple 1990). Selenium deficient lambs and calves may also suffer from a reduced growth rate and appear ?ill-thrifty.? Apparently normal merino lambs may also respond to supplementary selenium with increased growth rates and wool production. Finally, ewes and perhaps cows may suffer from infertility due to by embryonic mortality (Caple and McDonald 1983).
In southern Australia selenium intoxication is a risk when livestock are overdosed either by miscalculating the dilution of selenium concentrates, miscalculating bodyweight or through multiple administration of selenium containing drenches and vaccines. Gabbedy (1970) reported on eight cases of selenium poisoning in sheep. As might be expected, lambs were most commonly affected because they risked receiving an adult dose, because their weight may be overestimated and because oral selenium might be more available in lambs.
As little as 5.0 mg by intramuscular injection (1.0 mg/kg) and 10 mg orally (2.2 mg/kg) was sufficient to kill lambs. Diarrhoea occurred in the survivors. In another case, 50 mg killed 15 of 39 heavily pregnant merino ewes while 180 of 190, 2-6 week old lambs died following a dose of 64 mg. Most deaths occurred within the first three days but continued for 10 days.
Caple et al (1980) surveyed 708 sheep flocks throughout Victoria from early 1976 to mid 1979. While we are not aware of comparable data from NSW, many of their findings are directly applicable.
Caple et al found that blood glutathione peroxidase (GSHPx) levels ranged from 9 to 778 units. They classified GSHPx levels within each flock as less than 25, 25 to 50 and over 50 units. Blood levels of less than 25 units were regarded as indicating low selenium status. These values are comparable to blood selenium concentrations of 0.033 ug/ml, regarded as indicating deficiency in other studies. However, in New Zealand, Andrews et al (1976) used the lower figure of 0.02 ug/ml to indicated deficiency. Caple et al (1980) found that values were lowest in the spring and were on average 30% higher at their peak in the autumn. Autumn survey values were therefore corrected by 30% and winter and summer values by 15% to account for this seasonal variation.
Caple et al found that 12% of flocks sampled had mean GSHPx levels of less than 25 units when corrected for the season. We are unsure if the sheep properties surveyed were selected to accurately reflected Victoria?s sheep population or to reflect a geographic spread. However, 243 of the 708 flocks were surveyed during investigations of white muscle disease or ill thrift, potentially biasing the results in favour of deficient properties.
Most of the flocks with low GSHPx levels were in the higher rainfall areas while high levels were found in the drier areas of northern and western Victoria. Sheep grazing acid, granite derived and sandy coastal soils were most likely to record low GSHPx levels. This is consistent with NSW observations.
While superphosphate application is traditionally regarded as increasing the risk of selenium deficiency, Caple et al in examining the data from a stocking rate, superphosphate trail at Dunkeld in Victoria, found that plots in which superphosphate had been applied had similar levels of herbage selenium to those that did not receive superphosphate. However, GSHPx levels tended to be lower in plots that had superphosphate and higher stocking rates. This response could simply have been because superphosphate and higher stocking rates favoured an increase in the proportion of clover in the pasture. Clover at least in some assays had a lower selenium content than grasses
1. Survey of heifers
In 2006-7, 17 herds of beef heifers from across the central tablelands of NSW were surveyed to obtain baseline data on trace mineral status and parasite and pestivirus exposure (Watt 2007).
Nine of 17 herds of heifers tested had glutathione peroxidase levels of less than 20 U/gHb and three were from 20 to 40 units. Five mobs had levels regarded as satisfactory. GSHPX levels less than 40 units are considered marginal. Levels under 20 may be associated with clinical disease or production problems. None of the mobs tested had vitamin B12 levels that were either deficient or marginal. Selenium deficiency and pestivirus and liver fluke exposure were prevalent in the herds surveyed. Over 50% of herds had evidence of marked selenium deficiency (GSHPx levels below 20 units).
Despite this, herd fertility was generally excellent. There was little evidence of foetal loss. Most producers reported very few calves that appeared ?ill-thrifty? which might indicate pestivirus or selenium deficiency apart from the calves of heifers. Conception rates for females from those producers that pregnancy tested ranged from an average of 90% (range 80-100) for heifers, 91% for first calvers (range 83-100) and 96% (range 85-100) in mature cows. Producers were also asked to indicate the number of cows that calved from cows pregnancy tested in calf. This averaged 96% (and ranged from 80% to 100%). The mob with 80% calves from pregnancy-tested females was from a mob of Hereford heifers in which dystocia was a problem. All producers considered that abortions were a rare problem in their herd. Producers also indicated that they had few ?poor? calves. Those who reported poor calves commented that these were mainly in the heifers and they attributed this to poor nutrition.
2. Selenium response in beef heifers
In 2008, a selenium response trial was incorporated into an existing investigation into parasites in young cattle on a property near Panaura, south-west of Orange NSW. Five heifers were blood tested on 2 May. GSHPx levels ranged from 5-12 U/gHb (averaging under 10 units). On 12 June 2008, 58 heifers were randomly selected (from within each of the two parasite treatment groups) for treatment with barium selenate (Deposel, Novartis), while 59 were selected as controls.
|Average LWt at|
|Selenium group||Weaning||+3 mths||+6 mths||+9 mths||+12 mths|
Despite these very low GSHPx levels, which could be expected to decline even further in the spring, no liveweight response was detected.
3. Selenium response in weaned Merino lambs.
In March 2010, a selenium response trial was conducted near Tarana, about 40 km east of Bathurst. The property has improved pastures on granite-derived soils and a history of regular superphosphate applications. While the area is known to be selenium deficient, at the onset of the trial the merino weaners were only marginally deficient with GSHPx levels < 40 U/gHb. The results of this trial have been published elsewhere (Celi et al, 2010). However in brief the supplemented wether weaners had improved growth rates (the maximum difference was 2.86 kg) and fleece weights (2.75 versus 2.93 kg per head) and lower a mean worm egg count (WEC).
|Week||Group||Number||Mean (Eggs/gram faeces)|
4. Selenium and ewe fertility
The effect of selenium supplementation on ewe fertility in a flock of merino ewes from Bigga on the southern tablelands of NSW was also investigated. Previous blood tests indicated selenium deficiency. In January 2010, five weaners had average GSHPx levels of 57 (range 26-87), in July 2010 ten 18 month old merino ewes had average GSHPx levels of 42 (range 23-60) and on the 13th January 2010, at the commencement of this trial, maiden merino ewes had GSHPx levels averaging 67 units (range 34-94).
|Liveweight response||Scanned litter size|
|Se treatment||Number ewes||March||July||Growth rate||0||1||2||Mean|
These levels reflect only a marginal selenium deficiency. However, we saw no response in either liveweight or fertility.
A range of short acting selenium treatments is available for sheep and cattle. These include selenium fortified drenches and vaccines. Long acting options include injectible barium selenate and intra-rumenal capsules containing 5% elemental selenium. Judson and colleagues (Judson GJ, Ellis NJS, Kempe BR and Shallow M, 1991) showed that these long acting products were able to maintain adequate blood selenium levels in ewes on pastures for ?200 weeks? and to increase the selenium status of their lambs for 4-6 months. Rumen capsules are also available for cattle and increase blood selenium levels for about 9-12 months. However, Judson and Babidge (2010) found that barium selenate was also capable of raising blood selenium to normal for 2-3 years in cattle. They therefore recommended that cattle in marginal or deficient areas be treated with barium selenate every two years.
Our findings on the central tablelands of NSW are consistent with others. In selenium deficient areas, Merino weaners are likely to respond to treatment with improved growth rates, fleece weights and lower parasite levels. The response of selenium deficient crossbred lambs to supplementation is yet to be determined.
Ewes may respond to selenium supplementation with increased fertility but this response is less predictable and most likely to occur only on very deficient properties. However, selenium supplementation is clearly warranted on farms that experience white muscle disease. Fortunately, this manifestation of severe selenium deficiency now seems to be rare. Selenium responses in cattle are less likely than in sheep. Even highly selenium deficient young cattle may not show a growth response.
Despite the uncertainty of a response, with the advent of relatively inexpensive, efficacious long acting selenium treatments, we advise sheep and cattle producers on selenium deficient properties to supplement with long acting selenium. The ad hoc use of selenium fortified drenches , vaccines and supplements is less desirable because these products are short acting, risking both overdosing when used together and under dosing as they wane.