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Beth Wheeler – Dairy Cattle Nutritionist/OMAF - July 16, 2000
Reference: Ontario Ministry of Agriculture and Food

Haycrop forages, which are high in potassium, will increase the risk of milk fever when they are fed to transition cows before calving. This is because of the influence on the dietary cation:anion difference (DCAD). DCAD is the balance of the cations potassium (K) and sodium (Na) when compared to the anions chlorine (Cl) and Sulphur (S). The cations (K and Na) exert a positive charge, the anions (CL and S) exert a negative one. The pre-calving dry cow diet should be somewhat anionic (negative).

Of the four minerals in the DCAD, K levels are usually the highest and most variable in forages, and K often has the most influence on DCAD. Corn silage is low in K and grasses tend to be lower than legumes. First cuttings of legumes tend to be higher in K than later cuts.

The 1999 feed analysis summary from Agri-Food Laboratories for 1999 (Table 1) shows some very high K values for Ontario forages.

Table 1: Forage Summary – Agri-Food Laboratories, 1999

Forage	No. of Samples	K Levels (%, DM basis)
		Average	Minimum	Maximum
1st cut grass hay	310	1.94	0.52	3.7
1st cut grass silage	174	2.4	0.75	5.51
2nd cut grass silage	34	2.36	1.03	3.45
1st cut mixed hay	2271	1.98	0.68	3.81
1st cut mixed silage	2193	2.46	0.87	4.83
2nd cut mixed silage	1180	2.2	0.8	3.76
1st cut legume hay	276	2.25	1.07	3.68
2nd cut legume hay	1205	2.1	0.83	4.06
1st cut legume haylage	597	2.57	0.45	4.51
Corn Silage	1571	0.96	0.46	2.47

A survey of dairy nutritionists (feed dealers and private consultants representing over 1000 farms serviced) and farmers in Vermont, found that about 60% of the dairy farms have (or have the potential of) a high K problem. Table 2 shows how nutrition consultants and farmers in Vermont handle the problem. Which category are you in? How can you manage a high K situation on your farm? If you are in the "Don’t Manage It" category now, what is the plan for the future?

For some farms, corn silage is not a viable option. Many producers avoid feeding anionic salts due to their high cost and poor palatability. If forage levels are over 2.5% K or the total diet is over 1.5% K, you cannot feed enough anionic salts to overcome the problem. A good approach would be to grow (or buy?) "low K" forage. This was the least popular option in the survey.

A "low K" option requires growing, harvesting, storing and feeding this forage separately from other forages. In many rations, a feeding rate of only 2 to 3 kg per head per day of the "low K" forage is all that’s needed to dilute the K level down to acceptable values. If this is an option for you, the following considerations are important:

1. Select a field that tests low for K. Avoid clay fields as they have high K-supplying power.
2. If all your fields are medium to high in K, this won’t be an option this year but you can draw K down over several years. Plan ahead by designating the future low K fields and avoid K fertilizer or manure applications on these fields. If soil K levels are very high, it may take 8 to 10 years to return them to the normal range. If this is the case, consider purchasing "low K" hay in the meantime.
3. For low K forage you should grow timothy, bromegrass or reed canarygrass. Avoid orchardgrass, which is a stong accumulator of potassium. Avoid legumes such as alfalfa in seeding fields destined for pre-calving dry cow hay.
4. Test each cutting for K, and request wet chemistry analysis. Near Infra Red (NIR) analysis is quicker and cheaper but it is not suitable for ration balancing for minerals or calculation of ration DCAD values. Balance the pre-calving diet for the depressed intake normally expected before calving.
5. Harvest and store your "low K" forages separately, to allow year round access to this forage for pre-calving dry cows.

I remember a farm call I made to a "Century farm" a few years ago. This producer was feeding a grain mix containing anionic salts, but still having major milk fever problems. The forages had all been tested and the DCAD balanced. In summer, dry cows were kept in a turnout paddock that was a grassy field adjacent to the barn. We did some scissors cuttings on the pasture and found the K level was over 6%!! Can you imagine the manure application rate from 40 cows, 365 days a year, for 100 years?

High K in forage fields is a problem. The problem will only get worse with continued application of manure and fall potash. If you haven’t done any soil testing lately, plan to do some this summer. Samples should be frozen if they are not sent immediately. Remember the old saying…"if you can’t measure it, you can’t manage it"!

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