inside the grid add this tags
<ExportSettings ExportOnlyData="true"/>
<MasterTableView DataKeyNames="fldId" DataSourceID="DataSource1" AutoGenerateColumns="False">
<CommandItemSettings PrevFrozenColumnText="" ShowExportToExcelButton="true" ExportToCsvText="true" ExportToPdfImageUrl="true"></CommandItemSettings>
</MasterTableView >