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.

*Fe was not supplemented. Comment: Se was not detectable in pasture, but adequately supplemented.

The seasonal variation in tissue trace mineral concentrations is shown in Table 3.

Table 3: Seasonal liver mineral contents of dairy cows on pasture.

*Blood selenium in ng/ml; Comment: Within trace mineral, superscript differences indicate significant seasonal effects.

Copper intake compared to requirements (Table 2 compared to Table 1) appears low, but liver Cu compared to adequate liver contents (Table 3 compared to Table1) nevertheless appear to be well within the appropriate range. Season did not have a significant effect. Iron intake compared to requirement was excessive, but since liver Fe contents were apparently well within the required range, one suspects that the Fe absorbed from pasture was low because of known poor bioavailability. The seasonal effect on liver Fe contents was non-significant because of high between-animal variability, the latter which is also a function of the variable absorption of Fe.

Manganese intake compared to requirements appeared adequate, and liver Mn contents were within the adequate range. Season had a significant effect on liver contents, higher in summer and lower in autumn than in winter and spring. Liver and blood Se was well within the adequate range, with a seasonal high in winter. Zinc intake was comparatively high, but because liver Zn contents were at the lower end of the adequate range, low bioavailability or absorption is expected. Season showed a highly significant effect in the sequence spring < winter < autumn < summer.   

Stress effects were anticipated to result in inflammation which is classified as a non-infectious stress, heat stress being an example, which could implicate higher requirements. However, there were very little indication of either inadequate tissue levels of the trace minerals or stress. The comparatively high liver contents of Zn in summer and autumn is probably a reflection of high ZnO supplementation during this period. This was to counteract sporidesmin toxicity caused by the fungus Pseudopithomyces chartarum and resulting in Facial eczema primarily on ryegrass pastures during summer and autumn.

Conclusions: Although season did effect the trace mineral status of the dairy cows, the opposite of what was maybe expected was observed. The expectation that chronic inflammation associated with seasonal stressors, e.g. heat stress, would increase trace mineral demand to the extent of reducing trace mineral status was expected. This may have supported the need to improve trace mineral supplementation during such stressful times. However, there was no consistent trend with spring through to autumn, when high temperatures were observed. This may suggest that the trace mineral status of the study cows was adequate and the requirement of the immune system for activation not so large as to the extent of depleting mineral reserves, or it could simply imply that heat stress occurrences were infrequent and of too short duration to have a significant effect. One furthermore could have expected that the high Zn supplementation to counter sporidesmin toxicity, might have resulted in negative interaction with other trace minerals. However, there was no trend to that effect (Table 3). However, the high ZnO supplementation may affect macro mineral metabolism in bone and this possibility should be investigated in future.