Since 1983, the Pacific Northwest has managed over 50,000 acres of hybrid poplar primarily for the production of wood chips. Plantations have been established at high densities of 600 to 800 trees per acre with rotations of six to eight years. More recently, the management of newly established plantations has focused on the production of more valuable solid wood. This necessitates lower stocking rates (300 trees per acre) and longer rotations (10-12 years). At this wider spacing, substantial amounts of understory vegetation must be controlled during plantation establishment, over a longer period. Weed competition is detrimental to tree growth and provides habitat for vole populations.
GreenWood Resources conducted a trial of silvo-pasture for hybrid poplar with sheep in three age classes: age one, established with 7’ whips; age two established with 8’ poles; and age four established with 14” cuttings. We sowed a cover crop of forage oats and rape between the tree rows. Flocks grazed on 1-acre plots for a period of 27 days in August 2004. Sheep consumed forage and weeds in all three plots, but trees suffered unacceptable levels of browsing damage in the one-year stand. Analysis of tree measurements in the two-year-old and four-year-old trees showed no negative growth impact. Sheep performed well and remained healthy and vigorous while grazing on the plots.
This project sought to demonstrate a successful silvo-pasture system for hybrid poplars and sheep during the early years of the tree rotation. The challenge was to demonstrate that sheep could be successfully pastured in young stands without causing damage to the trees. We used whips and poles in stand establishments, which may help the trees withstand any sheep browsing.
To successfully demonstrate silvo-pasture for hybrid poplar that raises sheep during the early years of the tree rotation. The challenge was to demonstrate that sheep can successfully pasture in young stand without damaging the crop trees.
In each of the three fields, we measured tree growth inside and outside, using 36-tree plots, in a six-row by six-tree array. We established plots outside the sheep-grazing plot in the same six-tree rows within 100’ of the grazing plot border. Total tree height and height increments (the current year’s height growth) were measured in November and December 2004, using a telescoping height pole. We also tallied damaged trees at this time. Analysis of height data using General Linear Model statistical procedures showed no significant differences in total height or height increment at all three test locations.
We evaluated the condition of the sheep on August 20 and again on August 28, the day we moved the sheep from the plots. We substantially reached our target of having the sheep graze the entire month of August, grazing the animals for 27 days.
In the field of four-year-old hybrid poplars from cuttings, the forage oat cover crop performed well, despite heavy horsetail growth, and led to a heavy initial stocking rate of 20 sheep, the most of all three 1-acre plots. Heavy feeding reduced the number of animals after about 10 days. The trees showed no damage other than browsing of the lower leaves.
In the field of two-year-old hybrid poplars from whips, the sheep browsed leaves and softer branch tips to a height of 36”. A few trees showed some minor damage on the stem from chewing. The damaged trees were primarily within about 30’ of the watering trough. The sheep did not push over or break any trees. The sown forage oats have been well grazed, along with volunteer grasses, blackberry bushes and elderberry.
In the field with one-year-old hybrid poplars from whips, cover crops were the thinnest, leading to placement of fewer animals, initially, with a few more added in mid-August. The trees here were newly planted from whips and performed typically, putting the majority of their growth into developing root systems rather than the aboveground portion of the tree. Consequently, height increments are much smaller than the well-established trees in the other two fields. We tallied significant stem damage on 16% of the trees in the plot where sheep grazed, while no trees in the non-grazed plot showed any damage. Damage consisted of broken terminals.
Considering the forage available, the lambs did well. If more rape were present in the fields, we would expect much higher gains. Without rape and a limited amount of young grasses, the mature oats didn’t provide the protein quality and quantity needed for exceptional growth. By grazing these plots sooner with the oats in the boot stage, we would have seen better gains. However, the age of the lambs prevented us from turning in until August and high heat during the end of July and the first part of August pushed the maturation rate of the oats.
RECOMMENDATIONS AND NEW HYPOTHESES
Silvo-pasture with sheep in hybrid poplars can be successful in stands of trees that are two years old or older. Benefits to the trees include reduced weed cover and reduction of habitat for damaging small mammals such as meadow voles. Placing sheep in 1-year-old stands of hybrid poplar leads to levels of stem damage the tree crop can’t tolerate. Sheep performed well, but not as well as if the cover crop had been more balanced, with the inclusion of the rape forage.
Although the economics of establishing a forage cover crop specifically for sheep grazing compared to the revenue generated from grazing leases does not appear promising for the tree farm, using sheep to graze and help control native vegetation is a definite possibility.
Grazing sheep in hybrid poplar stands using only native vegetation, without the expense of sowing a forage crop, is the next logical step in determining if native vegetation can sustain the animals.
PRODUCER ADOPTION AND REACTIONS
Two local sheep producers, Magruder Farms, of Clatskanie, Oregon, and Scott Davidson, of Rainier, Oregon, ran a combined total of over 300 head of sheep on acreage of the Columbia Tree Farm, allowing the animals to graze on native vegetation. This is a direct result of the findings of the Silvo-Pasture trial.
Both Mac Stewart and Magruder Farms have responded favorably to the opportunity to run animals at the Columbia Tree Farm and are pleased with the forage condition and the performance of their sheep.
We held a field day on August 24, 2004, touring all three sites where sheep grazed. Eight people attended including representatives of the press, Oregon State University Extension service, the wool producing industry and local farmers and ranchers.
An article titled “GreenWood Resources, Magruder Farms Partner in Local Silvo-Pasture Project” appeared in the Clatskanie Chief newspaper, September 2, 2004.
An article titled “Sheep in the Poplars” appeared in the November 19, 2004, edition of the Capital Press, a regional agricultural publication.
We gave a presentation at the Ninth North American Agroforesty Conference on June 12-15 2005, in Rochester, Minnesota.