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ARE HIGH POTASSIUM LEVELS IN PASTURES A CAUSATIVE FACTOR OF MILK FLOCCULATION?

Discipline: milk; Key words: processing, casein integrity, Alizarol test, potassium, milk fat.

During the processing of long life or UHT milk, the milk is subjected to heat treatment. The heat treatment must be such that the casein complex can withstand the heat effect and sustain its integrity.  If it is not stable, the protein will precipitate and the shelf life of the milk will be compromised and may result in flocculation. This is unacceptable to the consumer, resulting in major financial losses. Milk buyers use the Alizarol test as a screening method to judge the heat stability and may reject the purchased milk if it does not pass the 72% Alizarol test. For processing to UHT milk, the test pass increases to 76%. In practice, the problem of unstable milk is more pronounced in milk produced by cows on pasture than in milk produced by cows in a TMR system. This may be associated with high potassium (K) levels in pasture, due to fertilization and other reasons. In relation to the hypothesis, the authors referenced below decided to establish whether high levels of potassium in the diet of cows on pasture indeed contribute to alcohol instability of milk.

Sixty Jersey cows of the Outeniqua Research herd between 30 and 150 days in milk were randomly allocated to three treatment groups of 20 cows each in a 3X3 Latin square design replicated 20 times (three periods of 20 days: 14 day adaptation and 7 days measurement period). Three concentrates treatments were compiled, respectively with low K: 1.05%, medium K: 2.93% and high K: 4.8%. Potassium carbonate was used to increase the K content of the concentrates. Cows grazed kikuyu pasture and were fed 6kg concentrate per day during milking (3kg per milking). Milk yield was recorded daily and composite morning and afternoon milk samples of each cow were collected on day 1, 3 and 5 of each measurement period. This resulted in 540 milk samples (60 cows X 3 samples X 3 periods). The alcohol stability of milk was determined using the Alizarol test at 72%, 74%, 76%, 78% and 80% alcohol. Milk fat, protein, lactose, milk urea nitrogen (MUN) and somatic cell count (SCC) were determined on all milk samples and the mineral content of the milk (Na, K, Ca, P and Mg) was determined on day 3 of each measurement period.

The average percentage Alizarol passed differed significantly (P<0.001) and was respectively 76.6a, 75.3b and 73.5c for milk from cows fed the low, medium and high K treatment. Milk fat % was 5.79, 5.84 and 5.64, milk protein % 3.71a, 3.67a and 3.51c , milk lactose %  4.55a, 4.52a and 4.29b, MUN (mg/dl) 15.5a, 14.1b and 12.0c, SCC (x1000/ml) 200a, 211a, 187b,  milk Na (mg/100g) 46.6, 48.7 and 44.3, milk K (mg/100g) 160a, 156a and 147b, milk Ca (mg/100g) 140a, 142a and 127b,  milk P (mg/100g) 89.9a, 87.8a and 80.9b , and milk  Mg (mg/100g)  12.5a, 12.5a and 11.5b,  for the low, medium and high K concentrate treatment respectively.

It was evident that the high K treatment significantly reduced the alcohol stability of the milk and also reduced the protein, lactose and MUN content. Furthermore, the Ca, K, P and Mg content of the milk was lower when cows were fed the high K concentrate compared to the control. The results convincingly suggest that mineral imbalances in the diet are associated with alcohol instability of milk and therefore possibly milk flocculation.

It is concluded that by reducing the intake of K, the problem of alcohol instability of milk should be alleviated. Farmers, therefore, should determine the mineral status of all paddocks on the farm. Additionally, they should limit K fertilization to the minimum required for plant growth, from paddocks with high K levels remove and ensile the material and avoid these paddocks, especially during warm weather. They should also not use chicken manure as fertilizer.

Reference:

J.L. Joubert& R. Meeske, 2017. Effect of potassium content in the diet of dairy cows on alcohol stability of milk. In Proc. of the 50th SASAS Congress, Port Elizabeth, 18-20 Sept. 2017.