Send data from a dynamic table via ajax

I have a dynamic table, where I can add or remove a row, when I add a new row I can send what is written by the form ... But not when I try to send via ajax, it only sends the first line. How would it be right to send all the new lines?

html code

<table id="table" class="table table-striped table-bordered table-hover" data-url="data1.json" data-height="299" data-show-refresh="true" data-show-toggle="true" data-show-columns="true" data-search="true" data-select-item-name="toolbar1" class="table table-striped table-condensed">
    <thead>
        <tr class="text-center">
            <th data-field="categoria" data-radio="true">Nº do Item</th>
            <th data-field="categoria" data-radio="true">Quantidade</th>
            <th data-field="categoria" data-radio="true">Preço unitário</th>
            <th data-field="categoria" data-radio="true">Regra destribuição</th>
            <th data-field="categoria" data-radio="true">Total</th>
            <th>-</th>
        </tr>
    </thead>
    <tbody>
        <tr class="linhas odd gradeX">
            <td><input type="text"  name="item_cod[]" id="item_cod" /></td>
            <td><input type="text"  name="quantidade[]" id="quantidade"/></td>
            <td><input type="text"  name="preco_und[]" id="preco_und" /></td>
            <td><input type="text"  name="regra_dest[]" id="regra_dest" /></td>
            <td><input type="text"  name="total[]" id="total" /></td>
            <td>
                <a href="#" class="btn btn-danger evento-btn-excluir tooltip-info excluir removerCampo"  data-toggle="tooltip" data-placement="top" title="Excluir" value="">
                    <span class="glyphicon glyphicon-trash" aria-hidden="true"></span></a>
            </td>
        </tr>
        <tr>
            <td colspan="6">
                <input type="text" name="qtdLinhas" id="qtdLinhas" value="1" style="width:40px;" />
                <a href="#" class="adicionarCampo"  data-toggle="tooltip" data-placement="top" title="Adicionar linha">                                               
                    <span class="glyphicon glyphicon-plus" aria-hidden="true"></span>
                </a>
            </td>
        </tr>
    </tbody>
</table>                    
<button  id="enviar" class="btn btn-success text-right" type="submit">
    <i class="fa fa-fw fa-save"></i> Salvar
</button>

ADD AND REMOVE LINES

$(function () {
 function removeCampo() {
 $("removerCampo").off("click");
 $(".removerCampo").on("click", function (){
  if($("tr.linhas").length > 1){
  $(this).parent().parent().remove();
  }   }); }  

$(".adicionarCampo").on("click", function (){
 var qtdLinhas = $("#qtdLinhas").val();
  for ( var i = 0 ; i < qtdLinhas ; i++ ){ 
   novoCampo = $("tr.linhas:first").clone(); 
   novoCampo.find("input").val(""); 
   novoCampo.insertAfter("tr.linhas:last"); 
   removeCampo(); } }); });

AJAX

$(document).ready(function(){    

    $('#enviar').click(function(){

        var item_cod = (document.getElementById('item_cod').value);
        var quantidade = (document.getElementById('quantidade').value);
        var preco_und = (document.getElementById('preco_und').value);
        var regra_dest = (document.getElementById('regra_dest').value);
        var total = (document.getElementById('total').value);
        var fornecedor =  (document.getElementById('fornecedor').value); 
        var data_lancamento = (document.getElementById('data_lancamento').value); 


        $.ajax({       
            type:"POST",            
            url:'./produtos/cadastrar-produto',                   
            data:{
                item_cod:item_cod,
                quantidade:quantidade,              
                preco_und:preco_und,
                regra_dest:regra_dest,
                total:total,
                fornecedor:fornecedor,
                data_lancamento:data_lancamento,                      
                _token: $('input[name="_token"]').val()
            },  
        });           
    }); 
});