Journal of the NACAA
ISSN 2158-9429
Volume 3, Issue 1 - July, 2010


Forage Production and Animal Stocking Rates in Thinned Douglas-Fir Forests

Angima, S.D., Assistant Professor, Oregon State University Extension
Green, S., Associate Professor, Arkansas State University
Reeb, J., Associate Professor, Oregon State University


By pre-commercial thinning Douglas-fir stands, canopy cover is reduced to a point where desirable livestock forages could thrive. In this study, the resultant forage was invigorated with nitrogen at 75 lbs/acre and forage production determined under a younger (25 years old) and mature (55 year old) stand of thinned Douglas-fir forest as a basis for determining livestock (beef) stocking capacity. Cumulative forage dry matter yields averaged 2.14 and 1.27 tons/acre for forages under the younger and mature stands, respectively, compared to the control (no trees) of 4.15 tons/acre. The control treatment produced approximately 27% more biomass than estimates from the USDA soil survey of 3 tons/acre in similar soil and climatic conditions. Current animal stocking rate for open areas in this region is one beef cow-calf unit to 2 acres. From this study we determined a new animal stocking rate (during the months of April to October) under thinned Douglas-fir forests of 4 and 6.5 acres per cow/calf unit for younger and mature stands, respectively. Using thinned forestland for forage production is another way to diversify and increase income for forest landowners.


Forage Production and Animal Stocking Rates in Thinned Douglas-Fir Forests
   In the Pacific Northwest, timber is a major crop bringing income to many small scale farms. The dominant and most productive tree species along the coast is Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco). Douglas-fir is planted at a density of 400-500 trees/acre. Depending on location and management, pre-commercial thinning at 15-20 years reduces tree density to 170-200 trees/acre with an average spacing of 10-15 feet. At this stage of growth, live tree crowns represent about 30-50% of the tree allowing sunlight to penetrate to the ground, thereby promoting growth of forages and other understory plants (Emmigham and Green 2003). Trees are typically harvested between the ages of 30 and 40 years, when they are fully mature.
   Other than pre-commercial thinning, income before harvest from forest stands is limited. However, planned forestry (in this case agroforestry - silvopastoral system) could provide earlier and greater economic returns if thinned forests can be utilized for forage production.  Agroforestry is the science of combining trees, shrubs, forages, grasses, livestock, and crops in innovative, flexible combinations tailored to the needs of landowners. Economic analyses by Kurtz (2000) and Kallenbach et al. (2006) have demonstrated that agroforestry can provide above average, long-term returns on investments. One of the five recognized agroforestry practices is silvopasture, which is the growing of perennial grasses and/or grass-legume mixes in a forest stand for livestock pasture. In this system, the trees not only provide long-term investment, but also provide the animals shade in summer and serve as windbreak in winter. In turn, the forage base provides feed for beef cattle, which ultimately provides livestock sales for short term income (Kallenbach et al. 2006).
   Just letting animals graze in a woodlot without both tree and forage management is not considered a silvopastoral practice because silvopasture requires intensive management of the perennial grass mixtures in the forest stand for livestock pasture. For example Clason (1999) found that timber and forage growth benefited from fertilization due to suppression of competing vegetation in a commercial loblolly pine plantation but Buergler et al. (2006), showed that without careful management, competition from many forage species reduces tree growth. In another study, Kallenbach et al. (2006) found that when fertilizer is applied to fescue (Schedonorus phoenix (Scop.) Holub ), the heights of black walnut trees (Juglans nigra L.) were reduced by 45% compared to a control without ground cover as a result of competition for nutrients and moisture.
   In temperate agroforestry systems, given similar soil and climatic conditions, forage growth is a function of light interception and temperature. Silvopastoral systems provide forages with an environment where both solar radiation and temperature vary spatially on daily and seasonal time scales (Sharrow 1991). The light quantity and quality reaching plants affect their morphology and dry matter allocation, provided temperature is adequate for active growth. Conifers have been found to reflect and scatter much less far-red light than hardwoods, therefore enhancing carbohydrate buildup in forages growing in the understory of conifer forests. These forages usually receive about 40% less photosynthetically active radiation (PAR) due to tree canopy structure but consistently produce greater than 60% of biomass compared to sites receiving 100% PAR (Feldhake et al. 2005).
   With light interception being the critical factor in forage production in thinned forests, our objective was to determine forage production biomass under existing younger (25 year old) and mature (55 year old) stands of thinned Douglas-fir forests that were customary fertilized once a year in spring with nitrogen (N) at 75 lb/acre as a basis for determining livestock (beef) stocking capacity in thinned Douglas-fir forests during the spring and summer months (April to October).
Materials and Methods
   This study was conducted on-farm in landowner’s field near Harlan, Oregon (44° 33' 07" N and 123° 48' 23" W). The soil type at this location is Eilertsen silt loam (Fine-silty, isotic, mesic Ultic Hapludalfs). Average annual precipitation is 92 inches/year with mean annual temperature of 53°F (Figure 1).
Figure 1. Monthly total precipitation and average temperatures at Alsea Fall Creek Hatchery, Oregon (10 miles from the study site), during 2007-2008. Historic long-term rainfall averages represent 30 years of data (source: Oregon Climate Service).
   The study was conducted under two forest settings: a younger stand (25 year old) and mature stand (55 year old) thinned Douglas-fir forests. Forest metrics including tree density, range and mean tree diameter at breast height (DBH), spacing, mean crown ratio, and crown cover are shown in Table 1.
Table 1. Tree density, range and mean tree diameter at breast height, spacing, mean crown ratio, and crown cover under two silvopastoral systems in a thinned, Douglas-fir forest in Harlan, Oregon.

Forest Metric
Younger Stand (25-yr old)
Mature Stand (55-yr Old)
Tree density (trees/acre)
< 100
Tree Diameter at DBH (inches)
Spacing (feet)
Mean crown ratio (%)
Crown cover (%)

   Three treatments consisting of forage plots measuring 10 by 33 ft were established based on location of forage plots in relation to edge of the woodlot and the aspect for each of the two tree age treatments, and a control where there were no trees. These were South-side – (SS), Center – (CC), and North-side – (NN). The SS treatments were at the edge of the woodlot on the south-most section that receives the most sunlight; the CC treatments were located at least 100 ft away from the SS treatments towards the center of the woodlot; the NN treatments were located at least 200 ft away from the SS treatments and received the least amount of sunlight. The choice of 100 ft was determined based on visual observations on the ground where the pattern of forage growth and vigor decreased substantially compared to the edge of the forest.   All the treatments were on south facing slopes at 5% slope, and each treatment was replicated three times for a total of 18 plots. The control plots were set out in the open without trees in the same configuration and on south facing slope. Urea was used as the source of N and was applied to all treatments at the rate of 75 lb/acre during the first week of April in 2007 and 2008 using an oscillating spout spreader. The addition of N was a general practice that landowners use every spring to invigorate forages after the long cold rains of winter. The site was gated and animals were not allowed to graze until after each harvest.
   The forage grasses were a mixture of perennial ryegrass (Lolium perenne L.) and orchardgrass (Dactylis glomerata L.). Cumulative annual forage yield data were collected for two years (2007-2008) and harvests were carried out May 25, and August 8, 2007 and April 21, June 25 and October 24, 2008, which were a day before the farmer turned in his animals for grazing. At harvest, forage was removed from a three feet swath, running the length of the entire plot, in the center of each plot with a flail type mower, collected, weighed, and recorded. Wet forage yields were adjusted to a dry weight basis by drying a subsample to constant weight at 134°F. All data were subject to analysis of variance using SAS (SAS, 1997). A Fisher protected LSD test procedure was used for mean separation at P<0.05.
Results and Discussion
Growing Conditions
   Climatic conditions at the study area favored rapid forage growth in spring and early summer, but the late summer dry period reduced growth substantially. The use of nitrogen invigorated the forages and they competed and grew well, suppressing weeds (visual observation). Harlan, Oregon has cool weather and abundant winter, spring and fall rainfall (Figure 1) that favors growth of cool season forages, even during summer with average high temperatures of 65°F.
Forage Yields
   Cumulative annual forage yields generally decreased with increasing distance from the edge of the woodlot from the south. As expected, forage yield from the control with no trees (4.15 tons/acre) was significantly greater than the treatments under both younger and mature stands of thinned Douglas-fir forests (Table 2).
Table 2. Average cumulative dry matter yield for perennial ryegrass and orchardgrass mix fertilized with 75 lbs N/acre under two silvopastoral systems in a thinned, Douglas-fir forest in Harlan, Oregon during the period 2007 to 2008.

Silvopastoral System
Under Younger Stand
(25-yrs old)
Under Mature Stand
( 55-yrs old)
---------------------------- tons/acre)--------------------------
South-side Plots (SS)
Center Plots (CC)
North-side Plots (NN)

Within columns, means followed by the same letter are not significantly different at P = 0.05 by Fisher’s protected LSD.
   Cumulative forage yields were not significantly different among locations for the forages growing under mature stands. However, for the younger stands, treatments at the south end of the woodlot had significantly greater forage yields than those towards the north end, but were not significantly different from those at the center (Table 2).
   These observations may be related to how plants invest nutrients in the different components of the photosynthetic apparatus. The investment of nutrients follows the pathway where plants will modify their biomass allocation to aboveground structures if carbon gain is negatively affected by low light levels (Fernandez et al. 2004). In this case, biomass allocation to aboveground structures is attained by increase in specific leaf area, which in turn increases light interception by orienting blades horizontally. Since forages in the SS treatments incidentally intercepted more light than those towards the center of the woodlot, they would allocate more nutrients to biomass buildup and consequently increase dry matter production.
   Silvopasture studies in the Midwest USA have shown cumulative forage production to be about 20% greater in open fields compared to pasture grown under trees, although forage quality was found to be marginally greater in the silvopasture system compared to the open field system (Kallenbach et al. 2006). The increased forage quality under shade can partially offset reduced forage dry matter under tree cover. Cool season grasses, such as orchardgrass and perennial rye continue growing productively under shade of up to 80% PAR (Lin et al. 1999). In our study, forage yield was reduced by 39%, 49% and 58% under the younger stand and 64%, 69%, and 75% under the mature stand for the SS, CC, and NN locations respectively, compared to forage growth without trees.
   Estimated mean stocking rate for beef cattle in western Oregon from soil survey is one cow/calf unit to 2 acres of pasture based on cumulative annual forage yield of 3 tons/acre. In this study, based on average cumulative annual forage production of 4.15 tons/acre without trees, 2.14 tons/acre under the younger stands, and 1.27 ton/acre under the mature stands, beef producers can set aside between 4 to 6.5 acres per cow-calf unit depending on whether they have younger or mature stands, respectively, during the grazing months April to October for animal production. This means that during spring, summer, and part of early fall, woodlot owners can utilize thinned forests for forage production to raise more livestock or hay for sale. Proper animal stocking rates will help landowners reduce overstocking that may help mitigate other limiting growth factors such as soil compaction.
   On the nutritive value of forages grown under shade, Kallenbach et al. (2006) showed that beef animals raised under silvopastoral systems had equal average daily gain (ADG) and gain per acre as animals grazed on open field systems with gain per acre being influenced by both cumulative forage production and forage quality. Kephart and Buxton (1993) reported that perennial cool season grasses like fescue and reed canary grass (Phalaris arundinacea L.) grown under shade produced more crude protein (CP) and less neutral detergent fiber (NDF) than the same grasses under full sunlight. Therefore, at these stocking rates we would not expect widespread feed supplementation for animals grazing under such silvopastoral system.
   1. Cumulative forage production for a perennial ryegrass - orchardgrass mix growing under a younger and mature stand of thinned Douglas-fir trees and fertilized with 75 lb N/acre was reduced by an average of 48%, and 69% respectively, compared to similar pastures in full sunlight. Forage production under trees was influenced by how much sun the forages intercepted which in turn may be governed by interrelated factors such as tree density, spacing, crown cover, and age of the stand.
   2. Forest landowners could potentially derive substantial future income from their thinned stands if they adopt well managed silvopastoral systems after commercially thinning their Douglas-fir forests. Animal stocking rates (during the grazing period April to October) of between 4 and 6.5 acres per cow-calf unit could be used for younger stands (25 year old) and mature stands (55 year old), respectively, under a silvopastoral system. Management under a silvopastoral system will allow production and supply of sufficient forage for proper animal weight gain and health. Success of individual systems will ultimately depend on location, management, and aspect of the property in relation to the sun. Re-seeding with forages after thinning and use of management intensive grazing are recommended under these configurations.
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