In systems where supplements make up a high proportion of the diet, varying nutrients such as protein, fibre, carbohydrate and fat may need to be considered, taking into account cow requirements and economics of purchasing specific supplements. With minerals, in addition to magnesium and calcium, grazing dairy cows may also require supplementary copper, cobalt, zinc, iodine, and selenium depending on the location and herd.
Determine the limiting nutrient
To determine if the cow requires more of a particular nutrient in the diet, the first step is to determine which nutrient(s) are limiting performance. The amount of milk produced is limited by the nutrient that is in shortest supply compared with requirement. Therefore, increasing the supply of non-limiting nutrients will not increase production.
During early lactation, the most common limiting nutrient in pasture-based systems is most often energy. The main energy source for the early lactating dairy cow comes from ruminal fermentation of carbohydrates. These can be divided into soluble sugars and starches (non-structural carbohydrates) or structural carbohydrates (cellulose and hemicellulose).
In pasture-based systems, trying to increase the amount of soluble sugars and starch in the diet is generally not cost effective. Although pasture does not contain 35% soluble sugar and starches (which is optimal for maximising milk production with TMR), the structural carbohydrates in good quality leafy spring pasture are highly digestible (70-85%) and degraded relatively quickly in the rumen. They supply similar energy to the non-structural carbohydrates.
Protein can become a limiting nutrient, if production levels increase above 2.4kgMS/cow/day. If this is the case, it is important to calculate whether energy or protein is limiting production, and the likely response, before choosing a supplement to use.
In addition to the main minerals that may be required in early lactation (e.g. calcium and magnesium) there are five trace elements likely to be deficient in grazing dairy cows and recommended for supplementation from 2-3 weeks pre-calving until 4 months post-calving.
These are copper, cobalt, selenium, iodine and zinc. Consult a veterinarian to determine mineral requirements in your herd.
Did you know?
Maintaining high quality pastures maximises the amount of highly digestible carbohydrates available for the cow? See Early spring management.
Unless total energy of the diet is increased, feeding a high starch supplement to a grazing dairy cow will not improve reproductive performance. See Body condition and nutrition for reproduction.