The grass is greener but the sheep are leaner
18 July 2007
Bryan L McLeod, Pro Ag Consulting
Plenty of fresh green pasture, so what's the problem?
Many times I hear the statement, “We have ample pasture but the stock are just not performing.”
Farmers will question their vets as to the reasons for the poor nutritional status of stock. Scouring and weight loss can be a major issue.
Tender wool is often a problem following the autumn rains, as is weight loss, lameness, and scouring.
Farmers who introduce irrigation will often see decreased individual animal performance as a result of increased pasture growth.
Mineral levels decrease
Many things happen when pastures are green and fresh and in a high growth stage. Firstly plant mineral levels, in relation to daily intake, decrease. Why?
[1] There is a greater volume of plant material due to the higher moisture content but plant dry matter, the portion without the moisture, is reduced. This means that in a grazing day, stock have a lower dry matter intake and a lower intake of minerals unless they can increase consumption. Cobalt (Vitamin B12) deficiency is a good example. Most deficiencies occur or become more acute during periods of fresh growth.
[2] Pasture protein levels increase rapidly as pasture growth increases as does pasture energy. Protein readings as high as 40 per cent are not uncommon during certain growth periods. So what’s the problem? The problem is that surplus protein needs to be processed by rumen (main stomach) microbes into ammonia so that it can be dispatched out of the system. Most animals only require 10 to 16 per cent dietary protein.
High protein pasture is also very digestible and requires little cud chewing and one of the roles of saliva is to maintain an ideal rumen pH of 6.9 to 7.0.
As ruminants ingest pasture, the rumen pH decreases but as they chew their cud saliva is passed into the rumen and the pH is restored.
However when pasture is too digestible and the diet lacks fibre, cud chewing reduces and saliva greatly reduces with the end result being a lower than desired pH in the rumen.
Side effects are reduced appetite, lameness, scouring, and trace element deficiencies. Thus we see stock chewing soil, trees, and gates in an attempt to increase their rumen pH.
Much of the energy in pasture is also lost to the animal when pasture protein is in excess as rumen microbes use this energy to process the surplus protein into ammonia so it can be dispatched into the blood.
For example, 4 ME of energy is lost when pasture protein is at 35 per cent. This is the reason stock require high levels of low protein high-fibre high-energy supplements.
The application of trace elements to pasture, as a foliar via boom spray or via irrigation water, is a very effective way of overcoming the side effects of high pasture protein.
In one case with fat lambs grazing irrigated pasture, daily weight gain increased 300 per cent once pastures were treated with trace elements through the irrigation water.
Elements to target are:
[a] Boron. Boron application as a spray will effectively reduce surplus pasture protein increasing available energy.
[b] Copper: A copper deficiency not only occurs when the level is low but also can be a result of interference by high pasture molybdenum, iron, zinc, and calcium. This means that if any of these minerals are in excess then the desired level of pasture copper needs to be increased to compensate.
[c] Cobalt. Related to vitamin B12. Needed in higher amounts when grazing fresh pasture. Essential for rumen activity, eliminating stress, and increasing weight gains.
[d] Magnesium: Normally low when pasture protein high. Suppressed by the natural increase in rumen potassium that occurs when pasture protein is high.
[e] Zinc, which is suppressed by a high iron intake that often occurs due to soiling of pasture.
[f] Manganese is required to counter the side effects of high protein.
There should be no impediments to optimum livestock production and health if pasture protein is balanced with suitable energy and mineral levels.