angular.module('app', []).controller('ctrl', function($scope){
$scope.devcustname = [
{customer_id: "3", customer_name: '', customer_email: "fas@gmail.com", contact_no: "23", address_line1: "6, f4"},
{customer_id: "4", customer_name: "ab", customer_email: "sm", contact_no: "12", address_line1: "R V Road, 10th cross"},
{customer_id: "5", customer_name: "da", customer_email: "3a@gmail.com", contact_no: "33", address_line1: "6, f4"}
];
$scope.a = {custid: "4"};
})
<script src="//code.angularjs.org/snapshot/angular.min.js"></script>
<div ng-app='app' ng-controller='ctrl'>
<select class="text-center form-control" name="custname" ng-model="a.custid" ng-init="temp = devcustname[0].customer_name || (a = {custid:''})" ng-change="fetchassocd(a)">
<option value="" ng-disabled="isDisabled">
-Please select a Customer name-
</option>
<option ng-repeat="a in devcustname | orderBy : 'customer_name'" value="{{a.customer_id}}">
{{a.customer_name}}
</option>
</select>
</div>