It is important to keep a close watch on trace mineral management of dairy cows, considering both the amount fed and the type of mineral. Any shortfall can compromise health, fertility and performance. Equally, if mineral feed rates are too high, money is being wasted, there is a negative impact on the environment and in some cases, over-supply of trace minerals can cause problems with toxicity. A mineral analysis should be carried out on all forages. Grass in particular is a variable source of minerals depending on factors including sward composition, season and soil type. Grass is commonly a poor source of some important minerals for dairy cows. To supplement dairy cows correctly and cost-effectively, one needs to know what is in every part of the diet. This is especially true for forages which typically supply over 50% of total dry matter intakes of a dairy herd.
The sources of minerals can make a huge difference. Cheaper supplements may contain higher levels of inorganic sources. Some of these are highly soluble and can interact with other ingredients in feeds and premixes as well as in the gastro-intestinal tract. Others are highly insoluble and not available to the animal. Sulphates, for example, are very reactive in the rumen which reduces their bioavailability. At the same time, the free metal which is released harms rumen microbes. The impact of sulphates in the rumen is significant because they have been shown to reduce the efficiency of fibre digestion1 so compromising the use of forage (see figure 1).
The most cost-effective way to feed minerals at any time of the year will be to take full account of all feeds in the diet and to supplement forages with bioavailable sources. Cutting back on minerals can be a false economy as any short-term gain will be wiped out by poorer performance, fertility and health.