April 2001
Intake more important
than quality
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%
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).