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Potassium In The Spring Watch Out Next Autumn!

Be careful with potassium (K) fertiliser this spring says Alex Goudy, Dairy Officer at DPI Warrnambool.

By Department of Primary Industries - 3rd November 2004 - Back to News

Being over-generous with potassium has the potential to cause milk fever and grass tetany next calving season. The worst case scenario of these disorders is the loss of cows, however even in less severe cases they can impact on milk production and therefore profitability.

Potassium is an important nutrient for pastures. Too little potassium in the soil means reduced pasture growth. Too much, and pasture plants can accumulate excess potassium and can cause a mineral imbalance in cows. A high potassium diet reduces the uptake of magnesium and calcium in the gut. Cows are most susceptible to mineral imbalances prior to and up to 3 months after calving.

A lot of potassium is removed in a hay or silage harvest, usually between 60-100kg per ha. So, many fertiliser recommendations in spring aim to give a boost to pasture growth using nitrogen, and also replace potassium, phosphorus, sulphur and nitrogen removed in harvested pasture. But, if soil potassium levels are already adequate, the pasture can accumulate additional potassium beyond what is needed for extra growth. This is called 'luxury uptake'.

And what is done with this hay or silage? Sometimes it is fed to the dry cows, and if it has high potassium levels, it can create a magnesium and calcium imbalance.

Most pastures growing on soils with adequate potassium levels (Colwel1 K levels between 200-250 mg/kg) have potassium concentrations of 2-3%. Pastures growing on soils with high potassium levels (Colwell K levels above 400 mg/kg) can have potassium concentrations of above 4% and these can cause grass tetany or milk fever.

So what’s the solution?

• Routinely soil test to determine the potassium levels of your paddocks.
• When trying to boost spring harvest yields, consider using straight nitrogen fertilisers if soil fertility is good.
• Do not apply potassium fertiliser in spring, unless soil potassium levels are less than 150-200 mg/kg and clearly limiting pasture yields.
• When building up soil potassium levels or replacing potassium removed in harvested forage, apply fertilisers containing potassium after the harvest, in late summer or early autumn.
• If potassium levels are high (>300mg/kg) consider cutting back on potassium fertilisers.
• Determine fertiliser requirements based on the nutrients needed, rather than an ‘off the shelf’ blend like 2 and 1.
• Cows susceptible to milk fever and grass tetany should be on a low potassium diet, fed rough hay or straw prior to calving, and kept off paddocks with high soil potassium tests, prior to and soon after calving. Magnesium supplementation pre and post calving will also reduce the chance of grass tetany if you are grazing susceptible pastures.

If these recommendations are followed, potassium fertiliser can be used with confidence on your dairy farm and there can be less of a worry about milk fever, grass tetany and dead cows. There is also potential to save some dollars by only using what your pastures need.

MANAGING THE WEANED HEIFERS

It is almost that time of year when the decision to wean the replacement heifers needs to be made. Many will do this simply on the basis of age and size, but in reality it should be based on their rumen development.

If the heifers are eating at least 1kg of pellets/concentrates and hay/straw or pasture then weaning should go relatively smoothly. If the heifer’s rumen is not fully developed at weaning then the post weaning growth rates will be affected. Therefore they will have less chance of attaining their target live weights at mating and first calving.

Undersized heifers have more calving difficulties, produce less milk and have greater difficulty getting back in calf during their first lactation. When lactating, they compete poorly with older cows for feed and because they are still growing, will use feed for growth rather than for producing milk. They are more likely to be culled for poor milk yield and/or infertility.

In order to avoid this situation, the heifers need to have target growth rates of 0.6-0.78 kg/day depending on the season. The live weight gain needs to be monitored by you as the manager, or the contractor paid by you on a live weight gain basis. It is only by doing this that you will determine whether the heifers are growing at the correct rate to achieve the target weight at calving.

It is easy to look at cutting costs on the dairy farm and to slash the cost of rearing heifers without thinking about the implications of that decision. At least 60% of the cost of heifer rearing occurs in the first year when management demands are much higher. Cost reduction strategies should only be applied to yearlings and older heifers when the long term effects are likely to be less dramatic.

If heifer growth rates are below target because of cost cutting or seasonal affects, some larger herds would mate them at 21 months to calve at 30 months of age or 18 months to calve at 27 months. This would be possible where there is more than one calving period each year. However, delaying calving by 12 months to attain target weight is not a profitable outcome for the following reasons:

• The potential milk production of dairy heifers is reduced by an increase in age at first calving. For example, if calving age was increased from 24 to 30 months and heifers produced 4,000 litres in their first lactation, each heifer would forego 2,000 litres of milk.
• Assuming a herd replacement rate of 25% for a 100 cow herd, delaying calving by 6 months would result in an extra 14 non-productive heifers competing with the milking herd for resources. If these resources were used to feed more cows, an extra 5 cows could be milked.
• Failure to reach target live weights at first calving can reduce lifetime milk production, by 20 to 23 litres/kg below target live weight at first calving. This amounts to a potential loss of say 1,050 litres for heifers with 50kg reduced live weights.

So you can see that heifer nutrition and management is important in achieving target live weights by 24 months of age.

Source: http://gippsland.com/

Published by: news@gippsland.com

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