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Experts are warning that magnesium deficiency in dairy cows and lactating ewes could be a real problem at turnout this spring because so much of the mineral has leached out from pastures this year. But according to David Thornton from Rumenco, newly calved suckler cows are especially at risk because these cattle do not receive much supplementary feed on many farms.

“Many suckler cows are coming out of the winter period in poor body condition, simply because they finished the 2012 grazing season in bad shape. The combination of last year’s wet, low quality end of year grazing and a housed winter on poor quality conserved forage means many suckler cows are ill-prepared nutritionally for the key grass staggers risk period,” he says.

He also advises farmers to be wary of suppliers claiming that multiple sources of magnesium in a supplement are better than a sole source of the mineral, urging the region’s suckler producers to focus on effective magnesium supplementation this spring.

“There are a number of different magnesium sources used by supplement manufacturers in their products, and some stress the importance of including a variety to boost availability of the mineral to the animal. But in fact magnesium availability is pretty similar across all the various common sources and true absorption in the animal is directly related to the magnesium content of the source, not its availability (see table 1).

“However, particle size in a magnesium source is important though. For example, when you compare the two magnesium oxide (Cal Mag) sources – in granular and powder form – the powdered Cal Mag delivers a higher magnesium absorption in the animal. And this is the source we use in the Supalyx Mag bucket, for example,” David Thornton says.

“Molassed, weatherproof buckets and blocks supplemented with the right source of magnesium should be fed free-access at grass. No troughs are needed and these types of supplements are available 24/7. They also prevent bullying and save feeding time and labour. Bear in mind too that magnesium is not very palatable, so being molassed, bucket and block products do help maintain the necessary intakes of this essential mineral.”  

Table 1: Sources of magnesium in spring suckler cow supplements

Magnesium Source          Magnesium Content        Magnesium Availability        True Magnesium Absorption

Magnesium chloride                120 g/kg                               89%                                      107 g/kg
Magnesium hydroxide             360 g/kg                               99%                                      356 g/kg
Magnesium phosphate            260 g/kg                               86%                                       224 g/kg
Ca Mg phosphate                   100 g/kg                               91%                                         91 g/kg
Magnesium oxide (powder)     510 g/kg                               84%                                       428 g/kg
Magnesium oxide (granular)    510 g/kg                               61%                                      311 g/kg
Magnesium sulphate               100 g/kg                               96%                                        96 g/kg      

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