Once the threat of metabolic issues around calving has passed, it can be easy to overlook the need for supplementing major mineral levels in the milking cow’s diet. Not balancing the mineral content of diets can be a false economy, which jeopardises both the health and productivity of stock.
The use of straights (commodity purchased feeds) and simple unmineralised blends of straights changes the mineral supply in the diet relative to a pasture-only diet and may make a significant difference to the total mineral supply to a herd and the farm as a whole. Palm kernel expeller (PKE) usage increased in 2017, at a time when compound feeds and blends also became more widely used – possibly as a way to dilute the contribution of PKE, as Fonterra suppliers prepared for the introduction of Fat Evaluation Index penalties this season. The growing shift from single straights to blends increases the opportunity to balance out some of the potential deficiencies of a pasture based diet, which may be exacerbated by the mineral content of some straights and conserved forages.
The mineral requirements of lactating dairy cows have been defined per kg of the dry matter intake by the National Research Council (NRC). The mineral content of pasture is not ideal relative to the nutritional requirement of dairy cattle, who are committed to depositing minerals in their milk daily with the intention that it will help safeguard the health and vitality of their offspring. Straights tend to be a poor source of calcium relative to the needs of milking cows, as indicated in the table. Grains are an especially poor source of calcium, so grain-based compound feeds like NRM Dairy Extra are fortified with added calcium. Farmers feeding rolled barley or wheat, kibbled maize, distillers dark grains or PKE run the risk that their cows will have to mobilise more bone calcium than would be necessary on a pasture-only diet. Whilst cows are good at mobilising body calcium in the short-term, if supplementary feeding is continued throughout lactation it may get harder for higher producing, older cows to replenish their bone calcium levels.
Many pastures contain insufficient magnesium to meet the needs of milking cows and high potassium, high protein and low sodium levels can reduce the magnesium availability. For most herds, supplementary magnesium is required through to Christmas and beyond for higher yielding herds. Topdressing magnesium oxide can be particularly inefficient when it is needed the most. Delivering magnesium in a blend or compound feed with some readily degradable carbohydrate improves absorption and helps mask the bitterness of magnesium oxide.
It is generally considered that high protein, vegetative pasture should have more than enough phosphorus for dairy cows. If milk urea levels fall through November it may be worth checking pasture protein and phosphorus levels, as sub-optimal results may indicate declining pasture quality. A straight like distillers dark grains may improve a diet by adding both protein and phosphorus.
Adding salt to blends is a great way to supplement sodium levels and also stimulate palatability, as salt is a good appetite stimulant.
For further information, contact your local Nutrition Specialist.
Article supplied by Dr Rob Derrick, Nutritionist