| Grazing is a main way to utilize grassland resources.Due to drought and overgrazing,more than 90% of grassland has been degraded to varying degrees in China.Therefore,it is urgent to optimize grazing management by the regulation of grazing intensity,grazing time and grazing method.Rest grazing,one of the main grazing management strategies in the world,is an important practice to balance animal demand and forage supply,however,there are few researches on it in China.Rest grazing according to sward height or herbage mass can improve the sustainability of grazing system,by sustaining the productivity of grassland and livestock and service function of the grassland.In this research,three seasonal rest grazing strategies were studied in improved meadow steppe with Bromus inermis in Hulun Buir,from June 15 to September 15(2015 to 2017).The rest grazing consisted of three seasons: spring(S1,from June 15 to July 15),summer(S2,from July 15 to August 15)and autumn(S3,from August 15 to September 15).The three rest grazing strategies were rest grazing in spring(R1,no defoliation in S1 and defoliation in S2 and S3),rest grazing in summer(R2,no defoliation in S2 and defoliation in S1 and S3)and rest grazing in autumn(R3,no defoliation in S3 and defoliation in S1 and S2),compared with continuous grazing(CG,grazing in all seasons)as control.To ensure sustainable utilization,the number of sheep in each plot was determined by residual herbage mass method,in which put-and-take stockings was applied to make the residual herbage mass consistent with the defaults.The following three contents were addressed:(1)the relationship between diet selection of sheep and the characteristics of the sward;(2)the impact of rest grazing practices on plant community composition,grassland productivity and plant diversity;(3)the impact of rest grazing practices on the nutritional quality of herbage on offer,nutrition intake and live weight gain of sheep.The results are as followings:(1)During the whole grazing season,the diet composition of Bromus inermis(40.4%)and Leymus chinensis(35.1%)were obviously larger than that of Potentilla bifurca(9.5%)and Carex(15.1%).Selectivity index of Potentilla bifurca was highest(0.62),followed by Bromus inermis(0.05)and Carex(0.17),and Leymus chinensis(-0.29)was lowest.During the whole grazing season,diet composition and the selectivity index of each species were strongly correlated with its quantitative characteristics in the sward(P < 0.01)and nutritional characteristics(P < 0.01),respectively.However,diet composition of each species was almost irrelevant to its quantitative characteristics in spring with highest sward availability,and selectivity index of each species was irrelevant to its nutritional characteristics in autumn with lowest sward availability.Therefore,diet selection of sheep was affected by both quantitative and nutritional characteristics of the sward and with the decline of sward availability,the influence of nutritional characteristics on diet selection of sheep decreased gradually,while the influence of quantitative characteristics increased.(2)During 3 years(2015-2017),stocking rate was similar among treatments.Due to the decreased(P < 0.01)pasture growth rate from spring to autumn and rest grazing in the first two seasons,herbage mass in R1 and R2 was significantly larger(P < 0.05)than that in CG.Rest grazing in different seasons did not influence dry weight proportion of both dominant species and non-dominant species in the sward(P > 0.05),due to the average herbage mass during grazing season more than 500 kg/ha and similar stocking rate among treatments.Pasture growth rate decreased with the increase of stocking rate within a season,indicating equal or under compensatory growth in this improved meadow steppe.Meanwhile,as diet composition of Bromus inermis and Leymus chinensis varied with seasons,dry weight proportion of Leymus chinensis in the sward in R2 was significantly(P < 0.01)larger than that in other treatments,while dry weigh proportion of Bromus inermis in the sward in R2 was significantly(P < 0.01)lower than that in other treatments.Rest grazing in different seasons had little(P > 0.05)impact on plant diversity,which was positively(P < 0.01)correlated with dry weight proportion of non-dominant species in the sward.Maintaining the ratio of dry weight proportion of dominant species to that of non-dominant species in the sward at 8:2 could help maintain grassland productivity and plant diversity.(3)The quality of herbage on offer was mainly influenced by plant community composition and pasture growth rate.Therefore,the quality of herbage on offer was similar among treatments,due to the similar pasture growth rate and plant community composition among treatments.Organic matter intake and digestible organic matter intake in R2 were significantly(P < 0.05)higher than those in other treatments,resulting in higher(P < 0.05)live weight gain per sheep in R2 than that in CG and R1.Specifically,the ratio of digestible energy to crude protein(0.099 MJ/g in spring,0.104 MJ/g in summer and 0.118 MJ/g in autumn)in sheep diet was higher than the value recommended by feeding standard(0.088 MJ/g)in 2016 with lowest pasture growth rate and poorest herbage nutritional quality.And,it was negatively(P < 0.05)correlated with apparent digestibility of nutrients.When the crude protein concentration of herbage on offer was lower than 8 %,organic matter digestibility decreased and sheep were required to supplement with protein feed.In conclusion,diet selection of sheep was influenced by both quantitative and nutritional characteristics of the sward and the extent of the influence was dependent on sward availability.R2 had a positive influence on the dry weight proportion of Leymus chinensis in the sward,but a negative influence on that of Bromus inermis.Nevertheless,rest grazing in different seasons did not influence dry weight proportion of both dominant species and non-dominant species in the sward,grassland productivity and plant diversity.R1 was not beneficial to nutrient intake of sheep but R2 was the best grazing practice for feed intake and live weight gain of sheep.In all,inter-annually alternating rest grazing in summer and autumn based on residual herbage mass for regulating stocking rate should be adopted to achieve sustainable utilization of grassland resources. |
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