The best way to fire onload event on dropdown to perform the same task as for onchange event is you can trigger the change function on script loading itself like below:
$(´#dropdown´).change(function(){
//right your code here
}).trigger(´change´);
For example you can refer following POC :
Dropdown.html:
------------------------------------------------
<html>
<head>
<title>Binding onload and onchange event on dropdown-anyforum.in</title>
</head>
<body>
<select id="country" name="country">
<option>India</option>
<option>Nepal</option>
<option>China</option>
<option>Japan</option>
</select>
<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js"></script>
<script type="text/javascript">
$(´#country´).change(function(){
alert($(this).val());
}).trigger(´change´);
</script>
</body>
</html>
Output:
------------------------------
India
Nepal
China
Japan