University of Vermont

Article Managing High K Forages for the Dry Cow

Sid Bosworth and Kellie Thomas, Extension Associate Professor and former UVM Forage Lab manager (presently with Poulin Grain), Plant and Soil Science Department, University of Vermont

In the past few years, high potassium levels in haycrop forage has been recognized as a potential problem for increasing the risk of milk fever if "high K" forages are fed during the last 3 to 4 weeks before calving. This is related to what nutritionist refer to as the Dietary Cation-Anion Difference (DCAD) which is the balance of the cations potassium (K) and sodium (Na) to the anions sulfur (S) and chloride (Cl). Lactating cows actually need a positive or cationic DCAD; however, dry cows during that last 3 to 4 weeks need a negative or anionic DCAD. On a practical basis, the dry cow forage needs to be less than 2% K on a dry matter basis. Levels above 2% K posing a greater risk.

Of the four minerals in the DCAD, it is potassium that is of the greatest quantity and most variable in forages and, therefore, has the most influence on DCAD. Forages can vary from below 1% to as high as 7% K on a dry matter basis depending on many factors including forage species, soil levels of K and prior fertility management, stage of maturity and time of year when harvested. The table below shows the average K levels of different forages reported from the University of Vermont Forage Testing Lab for 1997. Generally, corn silage is low in K, grasses can be low but have the potential to be high, and legumes tend to be medium to high. The grass hay was lower in K because most of the samples are from second or third cut (which tend to be lower in K compared to the first cut).

Table 1. Means and standard deviations of potassium (K) from 1997 samples analyzed
by the University of Vermont Forage Testing Lab.

Forage Type
No. of samples
% K
Grass Hay
1.9 ± 0.7
Grass Silage
2.5 ± 0.5
Legume Silage
2.8 ± 0.4
Corn Silage
1.2 ± 0.2

In a survey of dairy nutritionists (feed dealers and private consultants representing over 1000 farms serviced) and farmers in Vermont, about 60% of the dairy farms have or have the potential of a "High K" problem and, therefore, must manage their dry cow ration to assure a low or negative DCAD during that dry cow period.

There are many ways a farm can manage a "high K" situation. One approach is to dilute or substitute with lower K forage to reduce the DCAD to a more manageable level. Most farmers who do this use corn silage, which tends to be low in K. Another approach is to supplement with an anionic salt mineral. However, these tend to be unpalatable and require mixing in a TMR. A third approach is to manage a field for low K by appropriate cutting and fertility practices. This requires separate storage of the forage so it is only fed to the dry cows in the close up ration. According to our survey results in Vermont, about 45% to 50% of the farms feed corn silage or some other "low K" forage, 24% supplement with an anionic mineral salt, and less than 10 percent specifically grow, separate and feed a "low K" haycrop forage for their dry cows. About 25% to 30% don’t manage it at all.(Figure 1)

Figure 1. The percentage of Vermont dairy farms that use various strategies to manage "High K" forage according to a survey response from feed nutritionist.

Although there are not many farmers that are specifically growing and feeding "low K" haycrop forage, this is still a viable option especially for those that don’t grow or feed corn silage. Here are a few considerations if attempting this:

  • Select a field that tests low for potassium. Avoid clay fields because they have high K supplying power. If all your fields are medium to high in K, you can draw K down, especially on a sandy soil by fertilizing with N (and P if needed) to increase yields. This may take a couple of years depending on the initial soil level of K. Warning - high N fertilization will actually increase K content when soil K is adequate. Avoid potassium fertilizer or manure until your forage K levels are below 1.7%.
  • Grow timothy, bromegrass or reed canarygrass. Avoid orchardgrass, which is an accumulator of potassium. Avoid legumes such as alfalfa.
  • Cut the first harvest at early to mid head stage with an aftermath cut usually at 6 to 7 weeks later.
  • Test each cutting for potassium. Aftermath cuttings tend to be lower in K and that is what you may want for your dry cow feed. Request wet chemistry. Although NIRS analysis is good for crude protein and fiber, it is not accurate enough for minerals.

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Last modified May 26 2004 01:29 PM

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