| Grazing is one of the economic,efficient and green utilization of grassland.Sown grassland is one of the main food production bases in the world.The grazing utilization of sown grassland is an effective way to ensure the security of national food and ecology.Researches on grazing management of sown grassland is insufficient in China.Hence,a sown grassland-sheep grazing system was established in the Hexi Oasis.Based on 9years of grazing experiments,the characteristics of plant and livestock productivity and their formation mechanisms were studied,and a comprehensive evaluation was made from the perspective of the ecosystem.The main findings are as follows:1 The effect of grazing on the forage production of sown grassland(1)The effect of rational grazing on forage yieldThe annual average DM yields of barley,rye,wheat,alfalfa+tall fescue,alfalfa+bromus inermis,and alfalfa+bromus inermis+tall fescue pasture under grazing,were5.61?11.33?5.20?4.25?8.98 and 4.71 t/ha,respectively;the CP yields were 1.50,1.40?1.38?0.65?1.33,and 0.79 t/ha,respectively;the metabolizable energy(ME)yield were 146.44,147.4,136.28,31.77,54.54 and 40.15 GJ/ha,respectively.The mean DM yield for barley,rye and wheat monoculture pasture,alfalfa+tall fescue,alfalfa+bromus inermis and alfalfa+bromus inermis+tall fescue mixed pasture,varied by19.38%,46.10%,-11.79%(P>0.05),-3.44%(P>0.05),13.97%(P>0.05)and41.90%,respectively;mean crude protein(CP)yield changed by 204.32%,432.79%,324.38%,-2.03%(P>0.05),27.77%and 42.40%,respectively;the ME yield changed by 200.55%,239.91%,192.93%,-0.49%(P>0.05),24.73%and 117.93%,respectively.Interannual fluctuations in forage production were greater for wheat,alfalfa+tall fescue,and alfalfa+bromus inermis pasture.Grazing has a greater contribution rate to increase the forage production for annual pasture.The forge production of perennial pasture was affected by the interaction effect of utilization strategy and utilization years.(2)The effect of grazing on forage qualityThe annual average crude protein(CP)concentrations of barley,rye,wheat,alfalfa+tall fescue,alfalfa+bromus inermis,and alfalfa+bromus inermis+tall fescue grassland,were 11.53%,10.73%,10.60%,11.77%,9.63%and 10.43%DM,respectively;neutral detergent fiber(NDF)concentrations were 47.28%,39.26%,47.61%,47.99%,41.63%and 47.06%DM,respectively;acid detergent fiber(ADF)concentrations were 23.26%,20.72%,25.75%,22.05%,18.89%and 26.13%DM,respectively;dry matter digestibility(DMD)were 71.72%,74.19%,68.55%,70.78%,72.76%and 68.84%DM,respectively.Compared to grassland for hay harvest,annual variation rate of CP concentrations for grazing barley,rye,wheat,alfalfa+tall fescue,alfalfa+bromus inermis,and alfalfa+bromus inermis+tall fescue mixed pasture,increased by 0.83%,49.26%,24.51%,0.83%(P>0.05),46.03%and13.63%(P>0.05),respectively;NDF concentrations changed by(+indicates increase,and–indicates decrease)0.80%(P>0.05),-18.61%?8.89%,0.80%(P>0.05),-20.51%and 7.91%,respectively;ADF concentrations changed-0.99%,-29.78%,9.27%(P>0.05),-0.99%(P>0.05),-31.64%and 10.17%(P>0.05),respectively;DMD changed 0.24%(P>0.05),9.18%,-2.46%(P>0.05),0.24%(P>0.05),10.26%and-2.52%(P>0.05),respectively.Hence,the annual variations of quality for perennial pasture was greater than that for annual pasture.Grazing is an effective strategy to improve the forage quality of annual pasture.There was a significant interaction effect of grazing and utilization years on the forage quality of perennial pasture.(3)The effect of grazing on the utilization efficiency of sown grassland resourcesThe nitrogen use efficiency(NUE)of barley,rye,wheat,alfalfa+tall fescue,alfalfa+bromus inermis,and alfalfa+bromus inermis+tall fescue pasture under grazing,were 22.62%?35.52%?29.32%?9.32%?17.73%and 9.03%,respectively;the relative water use efficiency of dry matter yield(WUEDM)were 10.92,21.65,16.01,8.86,17.49,and 9.20 kg/ha/mm;the relative water use efficiency of crude protein yield(WUECP)were 1.21,1.90,1.56,1.02,1.94,and 0.99 kg/ha/mm.The mean NUE for barley,rye and wheat monuiculture pasture,alfalfa+tall fescue,alfalfa+bromus inermis and alfalfa+bromus inermis+tall fescue mixed pasture,increased by 34.97%,49.85%,128.34%,18.95%(P>0.05),40.88%and 10.30%,respectively;relative WUEDMincreased by 34.70%,1.18%(P>0.05),69.29%,16.49%,21.24%and-11.47%(P>0.05);the relative WUECP increased by 35.39%,49.67%,127.82%,19.31%,41.44%and 11.38%.Grazing enhanced NUE and WUE in annual pasture,while NUE and WUE in perennial mixed pasture were significantly affected by the interaction of year and grazing.The results from structural equation model(SEM)showed that precipitation had a direct positive effect on the forage yield and NUE of annual pasture;but no significant direct effect on the forage yield and NUE of mixed perennial pasture.The?0?accumulated temperature had a direct negative effect on forage production in annual pasture,and a direct positive effect on forage production in perennial mixed pasture;and had a direct positive effect on NUE of annual and perennial mixed pasture.Precipitation and accumulated temperature?0?had no significant direct influence on CP contents of forages.The NUE of mixed perennial pasture was also directly affected by soil temperature.Grazing mainly increases WUEDM and WUECP by increasing the dry matter yield and CP yield of annual and perennial pasture.2 The effect of grazing on livestock production(1)The effects on grazing behaviorIntaking rate,foraging rate,dry matter intake,intaking time/rumination time and energy consumption of grazing sheep changed with time and temperature in a"hump"curve.When the temperature was the highest during a day,the sheep would ruminate and have a rest.The intaking behavior of ewes was less sensitive to temperature.The chewing rate and chewing number per bolus and the interval of two boluses decreased with the time and temperature increasing.The intaking rate of sheep was positively correlated with the forage yield of annual pasture.The intaking behavior of ewes fluctuates slightly with the change of forage yield.The chewing number per bolus,chewing time,and the interval of two bolus in grazing ewes on an annual pasture fluctuates changes slightly with forage yield.The chewing number per bolus,chewing time,and the interval of two bolus of grazing ewes on mixed perennial pasture decreased with the forage yield increasing.Intaking energy and foraging energy consumptions on grazing barley,rye,wheat annual monoculture pasture,alfalfa+tall fescue,alfalfa+bromus inermis,and alfalfa+bromus inermis+tall fescue mixed perennial pasture accounted for 4.7%-4.9%,and 0.3%-0.8%of the energy intake.(2)The effect of grazing on rumen fermentation parameters and microorganismsThere was no significant difference between utilizations and sown grassland types in p H,ammonium nitrogen,butyrate acid,valeric acid ratio and A/P of sheep rumen fluid.The contents of total volatile fatty acid,the proportion of acetic acid,the proportion of propionic acid,and the proportion of isobutyric acid in the rumen fluid of sheep grazing on an annual pasture were higher than that of perennial mixed pasture by13.9%,3.0%,0.1%and 0.7%,respectively.The Shannon index of sheep grazing on perennial pasture increased by 17.32%compared with that of annual pasture(P<0.05).The relative content of Firmicutes and Actinobacteria in rumen fluid of sheep grazing on perennial pasture increased by 0.28%and 30.18%respectively compared with that of annual pasture(P<0.05).The relative content of Bacteroidetes,Proteobacteria and Tenericutes in rumen fluid of sheep grazing in perennial pasture decreased by 1.02%,40.72%and 26.69%compared with those of annual pasture(P<0.05).(3)The effect of grazing on serum physiological and biochemical indicatorsGrassland types and grazing management had no significant effect on the serum globulin and immunoglobulin IGG,IGA and IGM contents of sheep(P>0.05).Grassland type and grazing management had no significant effect on?-hydroxybutyric acid content,the free fatty acid content and alkaline phosphatase in sheep serum(P>0.05).The serum SOD content and GSH-px content of grazing sheep grazed on wheat grassland were the highest.(4)The effect of grazing on livebody weight gain(BWG)The BWG of sheep grazing on mixed perennial pasture was greater than that of annual pasture.The dry matter intake of rotating grazing sheep was positively correlated with the crude protein content of the forage;it had a"single hump"type change with the content of neutral detergent fiber and acid detergent fiber,and had a positive correlation with the metabolizable energy of the forage.The BWG of sheep increased first and then decreased with the change of temperature.The weight of ewes was less affected by temperature fluctuations.There was a positive correlation between the BWG and intaking rate of sheep in rotational grazing on annual pasture.The BWG and intaking rate of rotational grazing sheep in perennial mixed pasture show a"hump"type change.There was a negative correlation between the BWG and foraging rate of sheep in rotational grazing on annual pasture and the and that on perennial mixed pasture show a"hump"type change.The BWG and intaking time/rumination time of sheep grazing on annual pasture and perennial mixed pasture showed a"saddle"type change.3 Evaluation of productivity and ecological benefits of cultivated grassland-sheep grazing system(1)Evaluating the productivity of the grazing system by food equivalent unit(FEU)The FEU of forage and the FEU yield of sown grassland under grazing were higher8.62%and 2.98%than those of hay harvest production system.The FEU yield of grazing barley,rye,wheat annual monoculture pasture,alfalfa+tall fescue,alfalfa+bromus inermis and alfalfa+bromus inermis+tall fescue mixed pasture increased by0.40,1.43,0.15,0.07,0.07 and 0.21 times,compared to those of hay harvested.The FEU production capacity of grazing annual pasture was 15.20 FEU/ha,and the FEU production capacity of grazing perennial pasture was 21.23 FEU/ha.(2)Carbon balance analysis of grazing systemAccording to the carbon input and carbon output analysis methods of the four production layers and three interfaces of the grassland agro-ecosystem,the carbon balance of the grazing system was the carbon sink,and the carbon balance of the harvested hay system was the carbon source.The contribution rates of the carbon balance of the four production layers to the carbon balance of the grazing system were0.1%,84.6%,-0.5%and-17.0%,respectively;the contribution rates to the carbon balance of the hay harvest system were 0.1%,49.7%,and 0.0%.%and-51.1%.The carbon emissions of the animal production layer and the post-biological production layer of the grazing system and harvesting hay system accounted for 3.95%,96.05%,0%and 100%of the system's carbon emissions,respectively.The carbon balance of the three interfaces contributes 84.7%,49.8%,-0.5%,0.0%,-17.0%and-51.1%to the carbon balance of the grazing system and harvested hay system.The carbon emissions from the hay harvested system all came from fertilizer and irrigation inputs in the production process and carbon emissions from the processing and distribution of forage products,which about 3 times those from the grazing system.The animal production layer and the greenhouse gas emission from the grassland-livestock interface was need to be focued for reducing greenhouse gas emissions;the hay production system need to pay attention to the post-biological production layer and carbon emissions reduction during product processing and circulation Carbon output from the grass-livestock system-human activity interface.(3)Forage evaluation based on the perspective of ecosystemBased on the multi-scale and dominant factors evaluation in grassland agro-ecosystem,at the farm scale,the forage quality and yield of barley were better than those of rye and wheat.The CP content and dry matter yield of barley were significantly higher than those of rye and wheat.The NDF content,ADF content,and dry matter digestibility of barley were not significantly different from those of rye and wheat.The profitbility of barley was significantly higher than that of rye.The ecological benefit of barley was better than that of rye,and the CO2 equivalent emission based on dry matter yield of barley was significantly lower than that of wheat.There was no significant difference among barley,rye and wheat on the ratio of energy output and input,sustainable development index,and ecosystem service value.At the plot scale,the dry matter yield of rye and wheat were higher than barley.Rye had lower CO2 equivalent emissions based on dry matter yiled than barley and wheat.There was no significant difference among barley,rye and wheat on other nutritional quality,economic and ecological benefits.At the eco-region scale,rye had the higher economic benefits and the ratio of energy output and input than barley and wheat.The dry matter digestibility of rye was lower than barley and wheat.Wheat had the higher sustainability index than barley and rye.Therefore,based on the comprehensive evaluation,barley was relatively suitable for grazing in the research area.The optimized model of sown grassland-sheep production system which would have the high productivity of forage and livestock and low carbon emission in the arid area of northwestern inland was recommended to pay attention to the area ratio of annual pasture and perennial mixed pasture,the ratio of livestock grazing to harvested hay(means the combination of grazing and supplementary feeding). |
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