The monthly pasture cover – don’t die over it
Chris Boom – Northland
Assessing pasture cover on the farm does not need to be a daunting task, though it can be a fine line between spending enough time on the assessment to get good data and spending so much time it becomes onerous. Everyone will approach it differently, but we all need to find a technique that works and then do it on time every month. Here are some tips on getting accurate enough while still being do-able.
Why measure pasture cover
Farmax® software is the best management tool that I know of to provide analysis to back up stock policy and business decisions. However, to have confidence in Farmax predictions, you need a level of accuracy in the data put in. Reasonably accurate monthly pasture cover assessments are probably the most important data you collect. It gives you the start point future scenarios.
I think that doing a farm tour at the end of the month is the best way to get this cover assessment. This monthly discipline does also have other benefits. It forces you to check the back paddocks that you haven’t looked at for a few weeks and gives you ammunition to rethink your grazing levels, rotation lengths and stocking rates on different blocks. The dairy industry has pushed a weekly farm walk for a long time and once farmers get into the habit they realise the benefits.
Pasture cover assessment shouldn’t be a job for the most junior staff member. This should be the job of the top man who needs to understand the state of play on the farm and what feed is on what area. This is part of working smarter not harder.
Getting accurate
Don’t sweat the small stuff, you will never get it absolutely right. When assessing pasture cover on a paddock or farm you have to realise you are not really measuring, you are estimating. The trick is to get the estimate close enough so it is useful and easy enough that you will do it regularly and on time.
If you try to get accurate on every individual paddock, the job will kill you or you will kill it. Look at the big picture, each paddock is likely between 1 - 5% of the total farm area. It is more important to get the whole farm assessment close than die on trying to get each paddock assessment accurate.
Pasture cover is always measured in dry matter, water doesn’t count when it comes to stock feed demand. Kilograms of dry matter per hectare (kg DM/ha) is the standard measurement. Pasture height or clicks on the rising plate meter are meaningful if considering how well your stock are fed, but it needs to be converted to DM/ha to look at feed flows.
Full farm cover assessments
To get your monthly cover assessment, do a farm tour at the end of each month. Assess a selection of paddocks on the farm or block. Write these assessments into a note book and then average them to get your full farm monthly cover assessment.
Dividing the farm up into blocks based on different management blocks or terrain, and then calculating the total farm cover by averaging the blocks, weighted for their area, is a way of making sure one type of country doesn’t get over represented in the farm average. You can put these block areas into the pasture cover calculator in Farmtools.
If you want to get a bit more accurate then put your individual paddock names and areas in the cover calculator. This makes little difference if paddocks are fairly similar size, but if there are some big paddocks, you need to allow these to reflect more weight on the total farm cover than the small paddocks.
Visually assessing cover
Visual assessment is the quickest and most efficient technique, and is what I recommend. But you do need to calibrate yourself every month using some objective tool, such as a sward stick or rising plate meter. Some farmers feel confident enough to just visually assess their pasture mass without any tools to assist. This can be dangerous as we may (or may not) start right but usually drift (change) over time.
The use of a tool provides some constant that minimises drift. It also helps you adjust changes in pasture DM% that happen in different seasons. You can of course use the tool in every paddock, but this will slow you down heaps. If you have the time, great, but for most of us we have to compromise so the job is do-able.
Calibrating your eye by measuring pasture cover using a tool in a variety of paddocks at the start of your farm tour gets you in tune. Once you have a feel what 1200, 1800 and 2500 kg DM/ha look like, then continue on your tour and visually assess the paddocks as you drive through, over the fence or even across the gully. When you see something different and are unsure, stop and use the tool again.
You don’t need to assess every paddock; make it achievable. Your farm tour may take you into or past only 60% of the paddocks on the farm and as long as they are pretty representative of the feed on the farm then you won’t be far off.
Pasture assessment tools
The expensive tools (e.g. pasture probe, rising plate meter) give you an accurate cover assessment to the nearest kg (such as 1872 kg DM/ha). Yeah right! Just because it sounds accurate doesn’t mean it is. These are making an estimate and use a calibration that may or may not be appropriate for your pasture. These have usually been calibrated for dairy pasture conditions which tend to be open and erect and therefore they tend to underestimate sheep and beef pastures.
All tools need calibrating for different seasons. A pasture height of 10 cm in winter may equate to 2500 kg DM/ha while the same height in summer may be >3000 kg DM/ha. This seasonal change is primarily driven by an increase in DM% as the grass develops stem and the dead material builds up in the base of the sward.
Dealing with density differences
The problem with all tools is that none of them are very good at taking into account variation in pasture density. Density, especially the amount of material near the base of the sward, is a huge variable in getting accurate measures. A winter pasture that has a height of 10 cm may range from 1800 to 3200 kg DM/ha depending on the density. My preference for using the sward stick is somewhat because you can adjust for density when using it.
The tools
Sward stick
The sward stick is a simple tool that relates pasture height to pasture mass. This relationship was built up by Webby and Pengelly on sheep and beef hill pastures at Whatawhata (Waikato) during the early 1980s. These pastures were reasonably dense, predominantly ryegrass, browntop and clover which you need to keep in mind. If you have an open beef pasture then this tool will tend to overestimate.
You may be able to pick one up free at the local farm merchandise store (Ballance and Ravensdown both give them away). You measure the average pasture leaf height which, if there is white clover in the sward, is often indicated by the top of these clover leaves. This height is then converted to dry matter/ha by choosing the appropriate season.
The beauty of this technique is that it forces you to get down and dirty. While down there think, is this pasture more or less dense than normal. We call it visual estimation but it should be really called feel estimation because you need to get your hand into the pasture. Does it feel thick or thin? Does it feel hard and coarse (high dry matter percentage) or soft and full of water? With some thought and practice most people can get these estimates more accurate than other techniques because of the ability to adjust for density.
To assess a paddock you will need to measure more than one spot. Look around the paddock and chose a representative area. Maybe five measures will give you a feel for the average for the paddock. Of course more is better, but don’t kill yourself over it, make it do-able.
Rising plate meter
This is the chosen tool for the dairy industry. It mainly measures pasture height though it takes a bit of density into account. It is a great tool, however most are calibrated only for dairy pastures. It is not so useful on hill land due to gravity not being perpendicular to the ground. Rutted or rocky country also causes problems.
Pasture probe
This can also be a useful tool, however it too does not account for pasture density very well. The biggest limitation is that it is affected by the amount of water on or in the pasture. A dry pasture will read less than a wet pasture. The extreme of this is dry dead material that can read very low but be a very high pasture cover. Most are also calibrated only for dairy pastures.
Just do it
What ever way you approach your monthly pasture assessment, the most important thing is that you do it. Give the job the importance it deserves, but don’t die over it. Take time to build up your skill in calibrating your eye. Talk with others about their approach and then get into it with confidence.