Q. Why should I pasture sample?
A. Just like we soil sample to determine what nutrients are needed for plant growth, it makes sense that we should pasture sample to establish what nutrients our animals require to grow and perform. As pasture forms the majority of the diet in NZ livestock systems, understanding the nutrient content of the pastures on your farm gives you a major advantage. How?
Determining mineral deficiencies or excesses in your pastures and making fertiliser application decisions based on soil analysis alone is limiting. Minerals in the soil are subject to many interactions that can affect plant levels. These interactions and total levels will differ between farms, so that plant mineral levels will also vary considerably between farms.
Taking a pasture sample on your farm allows you to identify the actual levels of minerals being ingested by stock, so you can confidently determine what minerals they require to perform. Understanding pasture mineral levels also means you can confirm nutrient requirements for pasture growth, potentially saving you from unnecessary fertiliser application.
Q. Why take a 150mm soil sample over 75mm sample depth?
A. The commonly accepted sampling depth for soil sampling under pastures is 75mm. This is based on the assumption that this is the maximum root depth of pastures. However, if you are trying to encourage deeper root growth (to build soil carbon) or you have pasture species within your sward with deeper rooting depths (eg. Chicory), then understanding the nutrient content and availability at depth is important.
In these instances it is recommended that you take 150mm samples.
It’s also important to understand that nutrient content of the soil varies with depth. In uncultivated soils nutrients are typically higher in the top fraction. For example, an Olsen P test result will be lower in a 150mm sample compared to 75mm. See table below.
Q. What is DCAD and should I use anionic salts to reduce metabolic risk?
A. DCAD refers to dietary cation anion difference and is typically a measure of the proportion of the cations potassium (K) and sodium (Na), to the anions chlorine (Cl) and sulphur (S) in the diet. Physiologically, DCAD influences the animal’s acid-base homeostasis, Ca status around calving, and mineral element utilization.
Research and application of the DCAD concept shows that reducing the DCAD of a diet prior to calving will lessen metabolic alkalosis and reduce the risk for milk fever. Much of this research and application is based on Total Mixed Ration systems where the diet can be easily manipulated by adding anionic salts to reduce the DCAD (<250 meq/kg).
However, in all pasture diets typical of NZ livestock systems the DCAD is generally >300 meq/kg of dry matter. Strategies to reduce DCAD in a pasture-based system may not be of benefit. A negative DCAD can only be achieved by replacing pasture with low DCAD feeds (e.g. maize silage or PKE) and adding ionic salts to the diet (e.g. magnesium sulphate and magnesium chloride). This makes it a practical challenge as well as potentially uneconomic.
There are many factors contributing to Milk Fever risk including energy intakes, calcium, phosphorus and magnesium content of the diet. Understanding the mineral content of pastures on your farm means you can identify the risks and put in place strategies to reduce it.