He told a SIDE workshop on getting the most profit from pastures there was more than 60 years of science to back up what he was saying.
“We have really strong confidence in that science, but it’s showing us that farmers are only achieving optimal practice less than half of the time when it comes to post-grazing residuals and timing of grazing,” Chapman said.
“Revisiting this information shows how easily some operating profit can slip away if grazing management practices are not applied correctly. It’s difficult for farmers to see this slippage happening, which means it is hard to identify and correct the causes.”
Research done in the 1950s showed once the length of pasture stopped light reaching the base of the sward the mass of the pasture did not increase.
“When you get long, tall grass you also get dead grass at the bottom where no sunlight is reaching.”
He said ryegrass was termed a “three leaf” plant as it generally has a maximum of three live leaves on each tiller. After grazing, once the third new leaf has been produced the first leaf will start to die.
“Pastures are still growing new leaves at this point but the amount of new leaf being produced is cancelled out by the rate of leaf death, hence the net rate of pasture growth is zero.”
The rate of leaf growth is affected by temperature and soil moisture so varies seasonally and from year to year, but it is also affected by the amount of leaf left after grazing.
“When very little of the plant is left after grazing, there’s less leaf to capture the primary food source for the plant which is light energy so they have less energy to grow their first new leaf. That leaf that grows is also smaller in size so the second and third leaf again takes longer to emerge.”
Chapman said the optimal time to graze ryegrass pastures was when the third leaf had grown.
“Monitoring leaf stage gives us a simple and effective way to track what is happening. We want to stop dead matter building up at the base and making sure light is reaching all the leaves giving the plant the maximum amount of energy to grow.”
He said three ryegrass pastures at Lincoln were mowed, leaving behind residuals of 1150kg DM/ha, 1500kg DM/ha and 1850kg DM/ha, and allowed to grow for 45 days.
The area left with the low residual grew 1465kg DM/ha, the medium grew 1800kg DM/ha and the high only 1555kg DM/ha.
“Using the DairyNZ Forage Value Index figure for the economic value of pasture grown, the value of pasture growth lost due to low (overgrazed) and high (undergrazed) treatments was $100/ha and $74/ha respectively in the 45 days.”
He admitted best practice was often not easy to follow because of uncontrollable events such as wet weather causing paddocks to be waterlogged and potentially damaged if grazed.
Permanent fencing also caused problems with either too much grass in a paddock for the number of cows or too little causing either over- or under-grazing. As well, farmers wanted to keep autumn and winter-grown pasture to use in the spring for peak lactation and surpluses in late spring due to rapid pasture growth were often missed.
Chapman suggested mowing, either before or after grazing, could be used to improve pasture growth, especially when paddocks became “patchy”.
“There will be some patches that will be growing fast and others growing slow because of the amount of grass left behind in each patch following grazing,” he said. “These patches will continue to grow either fast or slow and so will not achieve optimal growth rates or quality.
“When patchiness becomes pronounced, this is a signal that there may be a case for mowing, either before or after grazing, to bring all of the area back to the desired state for high regrowth and maximum quality of feed at the next grazing.”
