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Herd health determined by metabolic status


In this month’s column we will start to look a little deeper into the real causes of metabolics at calving. While Dairy NZ has done an excellent job of explaining the use and benefit of magnesium before calving, many farmers still have questions about their cows’ health at farming. There are some interesting things to be considered in this space.

Why do farms differ so much in their metabolic health?

We know that the national average for hypocalcaemia (both clinical and sub clinical) is close to 40%, but it is the variation that is the puzzling part. If we take Farmer Con and Farmer Max that both run cross bred cows in the Waikato, both produce a little over national average at 400 kg/ms per cow, and 1200 kg/ms per hectare.

Both dust Mag Ox before calving. But Con hardly sees a downer cow and Max sees over 20%. What is going on?!

If you were to look at their production and cows and mineral practices you wonder how? The production stats would suggest they are getting similar amounts of energy. But there is much more to be considered.

Let’s start with the ground up. For starters, they might use different fertilisers. Con concentrates on well balanced fertilizer programs making sure his lime is up to date and maintains only moderate levels of potassium and phosphorus based fertilisers. He keeps nitrogen usage to a minimum and tries to maintain a healthy sward including clover. This probably results from a slower round meaning more mature grass matched with regular but small amount of supplementary feed. Importantly, his vitamins and minerals are in the feed in the right forms. Plus, he gets to make silage with the extra grass and uses it in the dry summers.

Ideally this approach results in a pasture with a more beneficial mineral content and regular feed intake with greater feed efficiency through the cow.

On the other hand, Max likes to keep things simple. After all he is a New Zealand grass only farmer. He uses urea as much as possible and does not worry about limestone fertilizer. He uses KCl (muriate of potassium) whenever he needs a pasture boost.

He runs a short round, not interested in legumes and never feeds any supplementary feed. That is until a summer drought when he pours in the bought in feed at up to 10kg a day until it rains in Autumn. If the cattle look like they are short of magnesium he puts magnesium chloride in the water and takes it out in Christmas, he can get them drinking up to 100g of magnesium chloride when it is hot.

This approach results in a pasture with poor calcium to phosphorus ratio, poorly available magnesium (Reeves 1996). Grass utilization in the cow is inefficient.

Ironically, both farmers use about 1mtonne of bought in feed per cow per year. Both farmers probably have similar fertilizer and feed input costs throughout the year.

So what is the difference? The key is in the availability of the minerals and vitamins in the pasture. What does the cow actually absorb from the diet?

The first thing to say is that this is a simplified overview. As nutritionists, we have to concede we have only touched the surface of what is really happening in the pasture fed cow. But if we keep it simple, there as some basic things that are pretty clear.

The first is the availability of calcium. A range of factors drives it, including vitamin D, magnesium and the ratio of calcium to phosphorus in the diet. Con is feeding a slightly more mature grass that also has a better mineral ratio. He has less issues with excess potassium and efficient utilisation which should give the gut more time to absorb key minerals.

Max has constant magnesium deficiency caused by excessive potassium and nitrogen. Further to this he is loading the cows with an anionic salt during spring, leading to loss of calcium from the animal (Oehlschlaeger 2014). Then when the summer drought hits his supplementary feed is lacking in calcium and often has excess phosphorus, not to mention an excess of fatty acids which can cause many issues.

When it comes to calving time Con’s cows have been able to replenish their skeleton, building reserves of calcium and phosphorus because they have been present in the right ratios. He never sees broken bones and the cows respond well to the most basic transition program.

On the other hand, Max’s cows never seem to be right. He occasionally sees a broken shoulder in his heifers (Dittmer 2016) and the milk fever is rampant. Spring is a procession of downers, followed by dirty cows, early stage mastitis and even poor fertility. His labor, health and involuntary culling costs are becoming a problem.

This is just a simplified example of how two relatively similar systems can have huge metabolic nutrient differences. There are many more extreme examples of this and we will try and cover off on some of them during the year.