TRACE MINERAL STATUS OF DAIRY COWS IN THE TSITSIKAMMA

Aim of investigation: To determine whether seasonal stressors affect the cow trace mineral status of dairy cows in the Tsitsikamma region of the Eastern Cape.

Experimental design: Trace mineral status was evaluated with 20 cows per farm on three farms over two years in spring (defined as Oct to Des), summer (Jan to March), autumn (April to June) and winter (July to Sept). The samples were taken at the end of each season.

Measurements: Blood and liver Se, liver Mn, liver Fe, liver Cu and liver Zn. The corresponding trace minerals were also monitored in pasture and water, in addition to Ca, P, Mg, K, Na and S in pasture, and TDS, Ca, Na, Cl, SO₄ and F in water. Only the results on trace minerals in feed and animal tissue are presented here.

Summary of main results and associated discussion: The animal-based results (liver and blood) are considered more reliable of the status. To estimate intake of trace minerals the requirements and expected DM intake from pasture and supplement were calculated, acknowledging that the outcomes are only indicative and shouldn’t be considered accurate, also because the bioavailability of trace minerals from pasture is comparatively low.                                                                                                                                         

The recommended trace mineral requirements and tissue reference ranges are shown in Table 1:

Table 1: Trace mineral requirements of 550-600kg dairy cows producing 25-30kg milk and liver if presumed adequate. [NRC, 1989; NASEM, 2021, adapted]

*blood selenium in ng/ml

The estimated trace mineral intake of a conceptual 550-600kg cow producing 25-30kg milk per day across seasons in comparison to needs as indicated in Table 1, are shown in Table 2:

Table 2: Estimated seasonal trace mineral intake of dairy cows from pasture and concentrate, assuming a pasture DM intake of 15 kg per day and 5kg DM intake per day from supplement.