And other thoughts from 20 years of grazing research
Jim Gerrish has learned a thing or two about grazing during some 20 years of poking around pastures.
For many years Gerrish has been the lead researcher at the University of Missouri’s Forage Systems Research Center at Linneus, which is generally recognized as the premier grazing research facility in the Midwest, if not the entire U.S. The FSRC has run a large number of trials attempting to measure forage and beef cattle performance—and how they interrelate—within a wide variety of grazing systems. Gerrish and his family also graze beef stockers and run a cow-calf operation on 260 acres in northern Missouri.
So, it would seem natural that Jim Gerrish has some opinions about grazing. He shared some thoughts about what really matters in grazing during a presentation at the 2001 GrassWorks Conference in Wausau, Wisconsin. Presented below are some aspects of what matters to Jim Gerrish based on that presentation, with some additional explanation based on FSRC work.
Intake on pasture is 75% availability, and only 25% quality.
Everyone knows that livestock will not grow or produce milk at optimal levels without adequate nutrient intake. Gerrish contends that in a grazing situation, it is far more important to have adequate forage available in the sward than to have the available sward be of high quality.
He notes that forage intake is a function of three factors: time spent grazing, biting rate, and bite size. The first two are largely beyond the grazier’s control. A ruminant is physically capable of grazing only 8-11 hours daily, and Gerrish notes that under ideal conditions, a ruminant can harvest all it needs in as little as four hours. Increased biting rates (30 per minute at the low end, 80 per minute at the high end) are often a result of the animal’s attempt to compensate for a lack of intake per bite (such as in a pasture with low forage availability). Thus, the primary job of the grazing manager is to maximize the amount of forage harvested with each bite.
A multi-year FSRC trial has compared performance of yearling steers grazing endophyte-free fescue, orchardgrass, bluegrass, red clover and birdsfoot trefoil pastures at four stocking rates: 300, 600, 900 and 1,200 lbs. per acre. The highest stocking rates produced higher-quality forage. However, net energy availability declined as stocking rates increased. That’s because intakes, as estimated by comparing pre- and post-grazing forage mass, also declined. At least for the steers rotated every one to three days, “there was no correlation at any point in the season between measurements of forage quality and estimated intake,” Gerrish said.
The best stocking rate thus appears to be “just a little bit on the high side of our traditional stocking rates,” Gerrish said. (Note: In the early 1990s, a Pennsylvania State University trial with dairy cows indicated that profits increased with stocking rates. However, those cows were supplemented with stored feeds when pasture ran short.)
It’s almost four times as important what the pasture looks like when you take them out as what it looks like when you put them in.
To Gerrish, “Leaving too short a residual in the summer is the biggest mistake graziers make.” Allowing stock to graze grass down to too low a height is a good way to go broke, he warns.
Some of this has to do with nutrient intake by the animal. “Bite size becomes limited as pastures are grazed to shorter heights,” Gerrish says. “We often focus management on ensuring that the pasture has reached a sufficient height to allow maximum intake, but then fail to move animals from the pasture before forage residual reaches a level where intake is restricted. At FSRC, we have found the correlation between daily intake and pre-grazing forage mass to be 20 to 25%, while correlation with residual mass to be 80 to 85%.”
Impact on root growth is another factor involved in residual management. Gerrish says that he’s starting to feel a bit uncomfortable with the old ‘take half, leave half’ grazing adage. That’s because the saying is based on a single-clipping trial with Rhodes grass, a little-used, warm-season species. The same researcher, F.J. Crider, also looked at the effects of various levels of leaf removal on smooth bromegrass and Kentucky bluegrass. In results published in 1955, Crider found that a single clipping of Rhodes grass that removed 50% of leaf mass caused a 2% stoppage in root growth. However, repeated defoliation of Rhodes grass produced 8% stoppage. Similar treatments with bromegrass and bluegrass resulted in 13% and 38% root growth stoppage, respectively. Sixty-percent defoliation greatly increased root damage for all three species.
So, in combination with some of the FSRC’s animal performance data, Gerrish says he’s starting to think that a 35-40% defoliation rate might be a better target. And that doesn’t even address the issue of drought tolerance.
In the Midwest, almost all the droughts we have are created through our management. We waste water.
Gerrish points to University of Nebraska research from the 1930s as evidence. In this study, three inches of rain were applied by sprinkler over a 90-minute period to pastures with a 10-degree slope. Both erosion and water runoff ballooned as pasture cover declined. Excellent, thick swards retained 95% of the water, while a poor stand with only 50% ground cover lost close to 75% of its moisture. Gerrish says that proper forage residuals maintain ground cover, root mass and soil organic matter, while reducing compaction. All of these factors are important to retaining moisture.
Droughts are better than monsoons.
Northern Missouri suffered an extremely dry summer in 1999. Forage yield in an ongoing variety trial at FSRC was down 65% compared to the previous, favorably wet, year. But mean dry matter yield per acre-inch was 43% higher in the dry year, and steer average daily gain was higher than in the previous three years of an ongoing trial. “It has been our experience over the years that grazing seasons with summer rainfall at least 20% below average usually produce our best individual animal performance. This response would seem to be related in part to the higher nutrient density of the drier forage,” Gerrish says. When compared to all the problems caused by extremely wet weather, Gerrish says he’ll take almost any drought instead.
Rest periods are difficult to quantify, but definitely needed.
Gerrish says they are a requirement if intra-paddock species diversity is to be maintained. Rest periods also reduce risk of compaction. Residuals also come into play here, as greater post-grazing forage mass provides greater flexibility in rest management compared to grazing very tight.
Turn animals in at 3,000 pounds of forage dry matter per acre, and take them out at no lower than 1,500 lbs.
Of course this varies based on sward’s density and dominant species. To maintain maximum intake, quality and plant re-growth capability, Gerrish says an animal should have to bite a plant only once to reach the proper residual. Low-growing species such as bluegrass and ryegrass should be grazed at six inches, with a two-inch post-grazing residual a good target if soil moisture conditions are good. Smooth bromegrass and alfalfa need something closer to a 10-inch turn-in height and a four-inch residual. Orchardgrass, timothy, tall fescue, red clover and birdsfoot trefoil can be grazed at 6 to 10 inches, with a target residual of three to four inches.
Gerrish offers this thumb rule: If you can’t see the eyes of a cow that has just been turned in to paddock, then low forage quality is probably limiting intake because the animals will have to take a second, lower-quality bite to reach the target residual. One the other end of the scale, forage intake is also likely being limited when the tops of the nostrils are visible while grazing (except for a very dense stand of a low-growing species).