jqgrid如何获取userdata的数据

jqgrid配置
. $("#VM_main_Grid").jqGrid({
                ...........
                colModel: [
                        { name: 'comcode', index: 'comcode', width: 2, sortable: false, editable: true, editoptions: { readonly: true } },
                        { name: 'upcomcode', width: 8, sortable: true, editable: false },
                        {
                            name: 'mobile', width: 5, sortable: true, editable: false, formatter: function typeformatter(cellvalue, options, rowObject) {
                                if (cellvalue != null && cellvalue != undefined) {
                                    var selectHtml = "<a href= \"/VM_main.aspx?upmobile=" + rowObject[2] + "\">" + cellvalue + "</a>";
                                } else {
                                    var selectHtml = "无";
                                }
                                return selectHtml;
                            }
                        },
                        { name: 'username', width: 6, sortable: true, editable: false },
                        {
                            name: 'yearsumpremium', width: 10, sortable: true, editable: false
                        },
                        { name: 'monthsumpremium', width: 10, sortable: true, editable: false },
                        { name: 'daysumpremium', width: 5, sortable: true, editable: false },
                        {
                            name: 'firstyearsumpremium', width: 10, sortable: true, editable: false
                        },
                        { name: 'firstmonthsumpremium', width: 10, sortable: true, editable: false },
                        { name: 'firstdaysumpremium', width: 10, sortable: true, editable: false },
                        {
                            name: 'secondyearsumpremium', width: 10, sortable: true, editable: true
                        },
                        { name: 'secondmonthsumpremium', width: 10, sortable: true, editable: true },
                        { name: 'seconddaysumpremium', width: 5, sortable: true, editable: true },
                        { name: 'statisticsdate', width: 5, sortable: true, editable: false }                      
                ],
                rowNum: 20,
                rowList: [20, 50, 100],
                pager: '#VM_Pager',
                jsonReader: {
                    userdata: "userdata"
},
                sortname: 'printid',
                viewrecords: true,
                multiselect: false,
                sortorder: 'asc',
                cellEdit: false,
                caption: '用户信息'
                
            });
 
读取:alert(jQuery("VM_main_Grid").jqGrid('getGridParam', 'userdata'))读取不到数据
alert(jQuery("#VM_main_Grid").jqGrid('getGridParam', 'userdata'))同样读取不到
是什么原因?
谢谢