jqGrid：Get custom jqGrid JSON data in loadComplete method

jqGrid JSON response:

{
    //Custom JSON data:
    "userdata":{"lastSearchTime":"2015/01/01:16:00:00"},

    //Normal JSON data:
    "page":1,
    "records":101,
    "rows":[...],
    "rowCount":10,
    "total":11
}

loadComplete: function() {
    var obj = $('#list').jqGrid('getGridParam', 'userData');
}