Response Of Plant Diseases To Beef Cattle Grazing Intensity In Hulunber Grassland

Grazing is the main utilization of native grassland,and forage disease is one of the limiting factors of grassland productivity.The present research was conducted to investigate the responses of the plant diseases to beef cattle grazing in the Hulunber grassland.The vegetation in the experimental area was Leymus chinensis and forbs,and other subdominant plant species also included Carex pediformis,Stipa baicalensis,Galium verum,Artemisia tanacetifolia,Bupleurum scorzonerifolium and Potentilla acaulis.Sanhe beef cattle（Bos taurus）was used in this experiment to set up six different grazing gradients,including 0,0.23,0.34,0.46,0.69 and 0.92 standard cattle unit（AU）/ha,with three replicates for each grazing gradient.The experimental site was established in the year 2007 and had been grazed for seven years before the present study.The relationship between plant diseases and grazing was systematically studied by field investigation and laboratory experiment for three consecutive years from 2014 to 2016.The main findings were as follows:1.Grazing had significant effects on grassland vegetation and the microenvironment（p<0.05）.The vegetation coverage and density of palatable grasses such as L.chinensis and S.baicalensis decreased with the increasing of grazing intensity,which can be expressed by the function of Y_coverage=-42.35X+34.34（R²=0.66,p<0.001）and Y_density=-178.19X+158.32（R²=0.81,p<0.001）.The coverage and density of edible herbs such as A.tanacetifolia also decreased with the increasing of grazing intensity,meeting the function of Y_coverage=-4.48x+7.23（R²=0.40,p<0.001）and Y_density=-7.6X+15.72（R²=0.54,p<0.001）.However,the coverage and density of small herbs such as P.acaulis increased with increasing of grazing intensity,which was in line with the relationship Y_coverage=1.48X+0.23（R²=0.49,p<0.001）and Y_density=11.34X+0.37（R²=0.52,p<0.001）.The surface microenvironment of grassland was also affected by grazing.The surface temperature at the 10 cm increased with the increasing of grazing intensity,in line with Y _temperature=3.33X+24.69（R²=0.46,p=0.015）,while the humidity had no significant relationship with grazing intensity（p=0.163）,which indicated that the effects of grazing on grassland vegetation were affected by selective foraging of beef cattle,and the changes of vegetation will further cause the variation of the microenvironment.2.In total,129 plant diseases were found on 55 plant species belonging to 17families and 42 genera.It mainly included leaf spots,rusts and powdery mildew,accounting for 41.1%,33.3%and 19.4%of the total number of diseases,respectively.To the best of our knowledge,six of the plant diseases were first identified in world.They were gray leaf spot of L.chinensis（Phaeosphaeria avenaria）,leaf spot of L.chinensis（Septoria nodorum）,leaf blight of L.chinensis（Bipolaris sorokiniana）,leaf rust of L.chinensis（Puccinia triticina）,leaf rust of Agropyron cristatum（P.elymi）and rust of A.tanacetifolia（Puccinia sp.）.Twenty two species of seed-borne fungi belonging to 19 genera were isolated and identified from the seeds of L.chinensis.Comparing with the isolated rates from no-grazing control treatment,the isloated rate of P.avenaria and S.nodorum was reduced by 24.1%and 25.0%,respectively,while the isolated rate of Epicoccum nigrum was increased by 80.0%.Follow-up testing verified the P.avenaria and S.nodorum were pathogentic fungi for L.chinensis,and E.nigrum inhibited the growth ofP.avenaria and S.nodorum,which indicated that grazing could affect the regenerative of L.chinensis and even the health of the community of the grassland through affecting the diversity and richness of seed-borne fungi.3.The effects of grazing on grassland disease varied with the grassland plant species.Compared with the no-grazing treatment,on average,the overall disease incidence of herbage and edible herbs such as L.chinensis and A.tanacetifolia in the0.23 AU/ha treatment decreased by 27.3%and 7.6%respectively,and the average disease incidence decreased by 613.2%and 345.3%in 0.92 AU/ha plots,respectively.The average disease incidence of small herbs such as P.acaulis was 13.4%in 0.92AU/ha plots,however no disease occurred in 0 AU/ha plots.The above results indicated that grazing directly decreased the disease incidence through herbivore foraging,but increasing the possibility of occurrence and prevalence of disease in remaining grassland plants.4.Grazing and disease had significant effects on biomass and nutrient content of L.chinensis（p<0.05）.Compared with 0 AU/ha plots,the dry matter of L.chinensis from 0.23,0.46 and 0.92 AU/ha plots decreased by 42.2%,90.5%and 339.5%respectively on average.Additionally,the crude protein content of L.chinensis from0.92 AU/ha plots,increased by 22.7%,compared with that from 0 AU/ha plots.Compared with healthy tillers,dry matter of L.chinensis tillers decreased by 45.6%and crude protein content decreased by 16.2%on average.The N:C,P:C and N:P of healthy and diseased plants were positively correlated with their relative growth rates,which was consistent with the growth rate hypothesis.This indicated that grazing might be affecting the content and proportion of nutrients in plants,and further affect the regeneration ability of plants.5.Grassland diseases were affected by grazing,plant density,coverage and other vegetation characteristic factors,and these effects vary within different plant species.Comprehensive analysis of the results showed that the grazing intensity had a significantly negative correlation（p<0.05）with the plant density,coverage and disease incidence of palatable grasses such as L.chinensis and edible herbs such as A.tanacetifolia,and gazing explained 62.0%and 54.5%of the disease incidence of two plant groups,respectively.Moreover,grazing can indirectly affect the incidence of diseases by influencing plant density and coverage,whose explained values were47.5%and 15.0%respectively.The combined effects explain values of grazing and vegetation characteristics were 66.5%and 48.0%,respectively.However,grazing intensities had a significantly positive correlation（p<0.05）with the plant density,vegetation coverage and disease incidence of small herbs such as P.acaulis,and grazing explained 43.0%of the incidence of small herbs.Similarly,grazing could affect the disease incidence indirectly by influencing plant density,coverage and other factors,and the explained values was 59.0%.The combined effects explain values of grazing and vegetation characteristics were 67.5%.This indicated that the effects of grazing on grassland plant disease were affected by selective foraging of beef cattle.In addition to the direct impacts,grazing also affected plant diseases severity via grassland vegetation indirectly.