We all know that grazing animals, especially sheep, are selective in what they eat. They graze selectively the best species of plants and also the best portions of the plant. The best plant parts are the palatable new growth.
WHY DO ANIMALS SELECT THEIR DIET SO PRECISELY?
Animals select new growth because it has higher nitrogen (protein) levels. The digestibility of what grows on the tips is higher. As a generalised comment, allowing for different plant species and different soils, there is 2.5% nitrogen in new leaf and 0.5% nitrogen in the stem. This is why animals select plants that are already over grazed. They have been eaten back to ground level and only have new leaf to offer. This explains why animals will always keep returning to the same plants under continuous grazing.
As grazing animals need from 0.8% to 1% of nitrogen for maintenance (to stay alive) and more for weight gain or lactating animals, it is in their interest to select the best diet available. To calculate the protein level of plants, multiply the nitrogen content by 6.25. Cattle are not as precise as sheep, but they are still selecting new growth when they take the top off growing grass.
AN EXAMPLE OF HOW PRECISE ANIMAL SELECTION CAN BE
Measuring grasses that livestock have eaten is not easy, so one day I decided to measure the diameter of the stem at the point of bite on Old Man Saltbush plants eaten by sheep in a 4,000 acre (1,600 ha) paddock. We were left in awe at the precision of their selection. The table below documents what they were doing.
Focusing on what was eaten, 49 bites out of 54 were selected from the stem diameter range 2.25 – 3.00 mm i.e. the new growth. They were basically avoiding stem >3.0 mm.
WHEN PERENNIAL GRASSES ARE AT MOST RISK
Perennial grasses are well adapted to drought but not to continuous defoliation. The most dangerous time for perennial grasses is a run of marginal years when stock eat all the new growth every time there is some rain. This results in root reserves being drawn on regularly with little replacement, and so some plants eventually die. This is what former CSIRO scientist David Freudenberger refers to as “the paradox of average years”. Green pick is ongoing so root reserves are at risk.
Plants, like animals, also have requirements. Often what is good for an animal is not good for plants, nor the pasture in general. An animal wants to keep a plant eaten down all the time so that there is a much larger percentage of new growth, while a plant needs to be allowed to grow to maintain health.
Animals are not forward thinkers so have to be managed. They will maximise short-term production to the detriment of long-term production. Plants and animals have evolved together and need each other, however carbon flows that are essential for paddock health and production, drop drastically if animals dominate plants.
The problem we have to confront is that the way animals select their diet following rain is not consistent with the way nature designed plants to function.
With regard to when it is best to harvest carbon flows, pastures should be rested after rain. In other words, graziers need to be harvesting only the surplus not the means by which a usable surplus is generated.