WINTER BONUS MAGAZINE 2026

D trains the soldiers for when there is a breach! Vitamin D3 is synthesised in the skin when the specialised sterols in the skin are exposed to the ultraviolet light from the sun. This vitamin is not yet biologically active until it has been transported to the liver and then to the kidneys, where it is converted to the hormone calcitriol. Calcitriol is critically important for calcium absorption and strong bones, but new research also suggests it has an important effect on immunity and cell differentiation. Specifically with transitional cows that have such high demands on their bodies, cows will lose much of their stored calcium to their milk and can suffer milk fever. Cows struggling with this low calcium, effectively have a low immune status and are vulnerable to secondary infections. It is critical to help the cow replenish her bone reserves and to keep her internal defences strong. Vitamin D also induces macrophages (immune cells) to produce antimicrobial peptides, which will prime the immune system by regulating neutrophils and monocytes, enabling the body to kill pathogens more efficiently. Should a virus, like FMD, breach the vitamin A-reinforced membranes, vitamin D has the immune system’s strike force ready to respond. In a time where we are more conscious of antibiotic resistance, vitamin D is a helpful tool to maintain the welfare, health, and longevity of your dairy herd. When this commander is missing in action, a vitamin D deficiency presents itself in cows with stiff gaits and swollen or painful joints due to slowed bone mineralisation. In calves, there will be a higher prevalence of rickets, and in milking cows, you would see a metabolic collapse during the transition period – milk fever. Vitamin D-deficient herds will also have sluggish immune systems, failing to respond to “normal” infections and vaccinations. To combat this deficiency, ensure that the animals are spending enough time in direct sunlight and fortify their diets with vitamin D3 through mineral premixes – it must be protected against oxidation as it can degrade when stored in hot, damp conditions. An injectable, concentrated form of bioavailable vitamin D will bypass the rumen and prime the macrophages and is best to inject 1–2 weeks before calving. Vitamin D needs magnesium to be a part of the reaction where it is converted into calcitriol, so if your commander seems to be failing despite vitamin D supplementation, then it could be worth double-checking your herd’s magnesium levels to see if your commander is missing its radio! Vitamin E – the cellular shield While vitamin D commands the troops, vitamin E and selenium provide the armour that keeps the troops from being destroyed in battle. Vitamin E must be provided through the diet in fresh green roughage, and since it is highly heat- and oxygen-sensitive, concentrations decrease drastically when the roughage is cut for hay or fermented for silage. It works as an antioxidant to protect the fatty membranes of cells. Vitamin E removes harmful free radicals and works alongside selenium, which forms part of an enzyme, to destroy toxic residues released by pathogens when they die, before they can destroy body cells. White blood cells, which are immune cells, neutrophils and macrophages, use oxidative bursts to kill pathogens (like the FMD virus). The vitamin E and Se shield protects cells from being damaged by their own weapons. You can see this clearly during calving, vitamin E levels naturally decrease at this time and leave the cow open to infection. Maintaining high enough levels of vitamin E and selenium through the dry period can reduce the incidence of clinical mastitis. Through high- quality feed or strategic injection protocols, it is important to keep the cellular shield intact so that the immune system is not just equipped, but is resilient enough to survive the fight – you can protect the milk quality, as well as longevity and welfare of your cows. If your shield is starting to look a bit rusty, the vitamin E and Se deficiency will yield white muscle disease in calves, an increase in incidences of retained placenta, and metritis in freshly calved cows, and immune- compromised cows that tend to spiral quickly from minor infections to clinical cases of mastitis. Strengthening the cellular shield is critical to defending against deficiencies and can be www.agribonus.co.za Winter 2026 BONUS 5