Planned grazing and allowing grasses to recover from grazing certainly works to restore pastures, improve ground cover, engage natural nutrient cycles and increase pasture growth. Lots of farmers around the world will attest to this. This doesn’t mean that it is easy to get it right though!
The following information draws on research from many years ago that a subscriber sent me a while ago, and the information is still so relevant. I believe it may be one factor in why not everyone gets the same outcomes from planned, rotational grazing. The information relates to what happens to the root system of a perennial grass plant when the top growth is grazed to different levels. This will obviously influence when we choose to move animals out of a paddock.
The trial included cool and warm season grasses of different growth patterns – rhizomes, stolons and bunch type growth. Grazing of these grasses was simulated by the manual cutting of their foliage. When half or more of the foliage of the grasses was removed, root growth was halted for a time after the removal (with the exception of one grass type). The time period for which the root growth was halted varied with the degree of the foliage removal. Foliage removal occurred in intervals of 10% – ranging from 10% to 90%.
Foliage removal impact on root growth cessation
A single cutting that removed most of the foliage resulted in complete root growth cessation for between 6 and 18 days. [private]The root growth stopped within 24hours of foliage removal, and “continued until recovery of the top growth was well advanced”1. I was well aware that recovering top growth relies on the reserves of the roots to get started after a graze. I am surprised however to learn that top growth can be well advanced before root growth even begins again.
With 0-40% foliage removal, none of the roots ceased growth. 50% or greater removal of the foliage resulted in increasing levels of root growth stoppage respectively, up to 90% removal which resulted in 100% root growth cessation.
Foliage removal impact on period of root growth cessation
The level of foliage removal also impacted the time before the roots began to regrow. Plants that had greater than 80% foliage removal had a percentage of roots that were still not growing at the end of the experiment 33 days later. 90% removal had complete root stoppage for 17 days, 40% of which were still stopped at the end of the experiment. Of the plants that had 80% foliage removed, complete root stoppage lasted 12 days, with 4% of these still not growing at the end of the trial.
Effects under set stocking situations
In one part of the trial, the foliage cuttings (or simulated grazings) was repeated 3 times per week (back to the level of the initial cutting) in order to simulate a set stocking situation, where plants might be grazed and regrazed during the stocking of that paddock. The experiment included foliage removal in 10% increments – from 10% to 90%.
The results were quite dramatic, but also not highly surprising given the effects that we know a set stocking situation can have on grasses. Under repeated cuttings, the % of foliage removal that impacted root cessation to 100% at the end of the trial came down to 60% foliage removal. It was not until the foliage removal was down to the 50% level or less that roots exhibited growth at the end of the study.
The study found that “the balance point in the relation of top reduction and root-growth stoppage was found to lie between the 40- and 50-percent clipping levels.”2 At these lower levels of removal – while there was still some cessation of root growth, it was only a percentage of the roots of any plant, leaving some roots to continue growing uninterrupted.
What does this mean for graziers?
There are many variables of real life that are not considered in this experiment, such as the fact that plants in the trial are cut with scissors – not pulled and trampled as they would be in a real life grazing situation. Nor is there the urine and manure present that may result on and around plants in a grazing situation. Surely there would also be some effect of time of year and climate on the regrowth activity of the root systems.
Despite all of this however, we can certainly take home some messages from the trial. This trial concluded that “the growing top cannot be reduced more than half without adversely affecting the functioning of the root system and the plant as a whole.”3 I would certainly not suggest taking the actual days to root regrowth as gospel in your grazing systems, but it is interesting to note that it seems that by leaving 50% and more of the plant behind in the growing season, we are much more likely to give our grasses a chance of continual root growth. This is likely to result in quicker regrowth and recovery of the plant, whilst also ensuring the longevity of the grasses in our pastures. The grazing of this portion of the plant would also mean the livestock are consuming a higher percentage energy (as the tips is where the energy is), which is likely to result in improved animal performance.
Continual root growth will also ensure constant plant sugars moving into the soil, which are necessary for the support of the ever important microbes of the soil.
It would be great to see a trial like this repeated for longer and taken through to see impacts on overall dry matter production.
An important observation is that many grazing paddocks are poly culture in nature and will have varying growing rates, and even within a species there will be variation of plant size. For some plants, one bite will be greater than 50% of the foliage.
Note that these trials were done during the growing season. I believe there is still a place for a graze of a large % of the grass plant foliage – particularly during a non-growing season, when more lignified matter can be grazed harder and crashed to the ground. We can also refer back to my blog with Dick Richardson, when I wrote about his message to ensure change in our grazing systems. So, if we are to graze in a manner of leaving more foliage on the plant, maybe there is merit in mixing this up every now and then, and have a more severe graze – so long as plants are allowed adequate time to recover afterwards.
And lastly, maybe we need to be digging a few more holes in our paddocks – before and after grazes – to give us a better feel for what we are looking at with regard to root growth and recovery. While this would just be subjective, I’m sure it can only be of benefit to get a feel for our own climates, soils, grasses etc. I know this is something we could certainly do more of on our farm.
References
The full trial information can be found at:
1,2,3. Crider, F.J. Root Growth Stoppage Resulting From Defoliation of Grass. Technical Bulletin, No. 1102, United States Department of Agriculture, Feb 1955.