Basic Cations (2)

Written by John Turner Consultancy

Magnesium:

Magnesium deficiency is 'hypomagnesaemia' (low blood/body fluid magnesium) or 'grass tetany' (convulsions and profuse salivation or frothing at the mouth).  Subclinical deficiency tends to affect animal behaviour with increased excitability.  Death can be rapid for clinically affected animals.

Hypomagnesaemia can occur quickly (within a couple of days) because bone magnesium reserves are not able to be quickly mobilised to maintian blood magnesium levels.  Mature lactating cows grazing low magnesium content feed with a high potassium level are most likely to be affected.

The magnesium content of pastures is affected by soil temperature.pasture content reducing by 0.01% per degree reduction.

Sodium:

Sodium deficiency is often overlooked because the effect is not as dramatic as milk fever or hypomagnesaemia.  However it has been estimated that up to 30% of Waikato dairy herds have reduced milk production due to sodium deficiency.

Low sodium levels (less than 0.1%) are typical of pumice soils in the Central North Island.  Northland,Taranaki, Manawatu and the South Islands West Coast often have high pasture sodium levels due to salt spray being carried on the prevailing westerly wind.

Plant growth research has shown that many plants can utilise sodium as a nutrient, especially where soil potassium level is low.  Sodium is thought to reduce luxury uptake of potassium by plants where soil potassium level are high.  Excessively high levels of sodium in the diet has the same effect as high potassium as a predisposing factor for the incidence of milk fever.  Dietary Cation Anion Difference (DCAD) is used to estimate the combined effect of dietary potassium and sodium on dairy cows.

Calcium and Magnesium Supplements - How Much?

Where pasture treatment is considered, it is interesting ot calculate the requirement to raise the concentrations from low to adequate:

                         Increase required          Treat 2000 kg DM/ha                    per 16 kgDM

Calcium           +0.2%                             4 kg Ca or 10 kg Lime                  80 grams lime

Magnesium     +0.1%                             2 kg Mg or 3.7 kg Mag Oxide      30 grams MgO

Sodium            +0.07%                          1.4 kg Na or 3.5 kg Salt                28 grams salt

These calculated rates represent 'worst case' scenarios, several factors influence the proportion of dietary calcium and magnesium that is absorbed through the ruminant gut.  Do not use this information as a recommendation.

Seasonal Changes in Ryegrass Pasture Nutrient Levels:

Ryegrass based pastures have some growth stage variations due to seasonal growing conditions, the seasonal changes could be similar to the table below.

          Spring/Autumn               Early Summer              Summer Dry

          Lush leafy                       Stem elongation           seed  head

N       4.5%                                3%                                 1.8%

K       3.2%                                2.8%                              2.3%

Ca     0.4%                                0.25%                            0.2%

It is coincidental that the stress period for spring calving cows is when the relative levels of basic cations are most unfavourable.  At around mating time, 3 months after the start of calving, the nutrient balance is better suited to dairy cows and this will influence the cows requirements for nutrient supplements.

Management of Dairy Cows around calving:

The first step towards minimising metabolic problems is to maintain reasonable cow condition through winter, a condition score of 5 is considered optimum. Fat cows are at risk of developing milk fever at calving.  Potassium fertiliser should not be applied during winter/spring if there is a risk of inducing metabolic stress. Magnesium supplementation should ideally be started at a high rate about 3 weeks before calving is expected to start. This increases the magnesium status of cows and provides a small calcium 'stress' which stimulates the process of calcium mobilisation from bones. Lime application to pastures should be avoided pre-calving where there is any chance of direct ingestion of lime dust.  An old fashoned  approach was to starve the cows for a short time pre-calving to create stress and calcium mobilisation, this is not recommended.

After calving, the rate of magnesium supplemented is often continued until mating or until bloodtest levels and feed analysis data shows that this is not required.

Sodium (salt) may be included in a supplementation mix if sodium analysis of the total diet is lower than optimum for lactation.

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