Graduate Thesis Or Dissertation
 

Effects of sheep grazing to control weeds in a pine plantation on weed reproductive success, trees and sheep performance

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  • Use of sheep to control weeds in a ryegrass-white clover pasture in which Knob Cone-Monterrey hybrid pine trees had been planted was investigated in Corvallis, Oregon during 1997-1998. Treatments consisted of an ungrazed control (C) and grazing applications to achieve 25 (L), 50 (M) or 75 (H) percent utilization of the understory vegetation. The treatments were applied three times between May and July of 1998 (T1, T2, and T3). Cover and phenological stage by plant group was measured after each treatment application. Two species of interest, Himalayan blackberry and bull thistle, were also studied independently of the other forbs. Their growth was measured at T1, T2, T3 and also in October of 1998 (T4). All yearling ewes were weighed at the beginning of the trial, before T1, and again after T3. Sheep used in the trial were also weighed after T2. Initially understory vegetation covered about 90% of the ground, approximately 20% of which was perennial ryegrass (RYE). Incidence of other perennial grasses (OPG) remained constant at about 20% where ungrazed but declined from 20% to 10% during the study in all grazed treatments. Annual grasses (AG) performed similarly to OPG, remaining at 25% in C throughout the trial but declining to 8% or less in all grazed treatments by T3. Forb cover was variable throughout the site, ranging from about 20 to 50% before initial grazing. Grazing at T1 reduced forb cover by 44% (L) to 80% (H) from initial levels. Forbs did not fully recover by T2 and constituted about 25% of the cover in C and L and <10% in M and H. Grazing at T2 reduced forb presence by 25% in L, with little change in M and H due to the low presence of forbs in those treatments. Late season forbs began to appear by T2 and were dominant by T3 when grazing reduced total forb cover to 15% in L and <10% in M and H. Grazing also delayed maturation of all plant groups, with some affected more than others. All grazing treatments resulted in delayed RYE maturation compared to C (p<0.01). By T3 80% of RYE plants were reproductive in C, while less than 50% were scored as reproductive in grazed areas. Almost 90% of OPG in C had set seed by T2. By comparison, in the grazed treatments an average of 53% of OPG plants had set seed prior to T2 grazing and this was reduced to 13% post-grazing (p=0.0001). The more intensive grazing treatments reduced seed set compared to L (33% vs. 3%, p=0.001) but there was no difference in seed set between M and H. By T2 all AG sampled in ungrazed, control cells and 60% of AG plants in L had set seed. In M and H most were still flowering with nearly none having yet set seed (p<0.01). By T3 there was very little AG present in any of the grazed cells. At T2 about 50% of the forbs had set seed in C while most of the remainder were still vegetative, reflecting a transition in species present from early season species to late season species. Late season forbs were actively growing at T3 and nearly all forbs in C were reproductive. In C the proportion of forbs having set seed was about 40% at T2 and T3, compared to 25% or less in any of the grazed treatments at either T2 or T3 (p<0.01). Grazing had no effect on the number of bull thistle flowers produced per plant but did reduce plant height. Thistles in C averaged 71 cm in October (T4) compared to 42 cm in L and 32 cm for M and H (p<0.01). Himalayan blackberry cane length was also affected by grazing, reaching 81 cm in length in C by T3 but was reduced to 31 cm in L, 23 cm in M and 13 cm in H (p<0.01). No increases in cane length were observed after T3 in C. However, the canes continued to grow between T3 and T4 in the grazed treatments to achieve lengths of 37 cm (L), 41 cm (M) and 27 cm (H). Intense grazing at T1 resulted in browsing of nearly all available pine branches in H compared to <30% browsed in L and M. Tree browsing in L was slightly higher at T2 than at T1 and was 25% higher in M. No additional browsing occurred at T3 in any treatment (p>0.3). Sheep performance was not affected by the grazing treatments. There were no significant differences in weight gain among the three groups of yearling ewes used in the trial. Moderate levels of sheep grazing can be an effective tool to reduce competition from many undesirable species including Himalayan blackberry.
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