farm/views/account_invoice.xml

<?xml version="1.0" encoding="UTF-8"?>
<openerp>
<data>
    <record model="ir.ui.view" id="farm_invoice_form">
        <field name="name">farm.invoice.form</field>
        <field name="model">account.invoice</field>
        <field name="inherit_id" ref="account.invoice_form"/>
        <field name="arch" type="xml">
            <field name="quantity" position="before">
                <field name="animal_qty"/>
            </field>
        </field>
    </record>
    <template id="report_invoice_n_animals" inherit_id="account.report_invoice_document">
            <xpath expr="//table/thead/tr/th[1]" position="before">
                <t t-if="o.invoice_line[0].animal_qty">
                <th class="text-right">Num. Animals</th>
                </t>
            </xpath>
            <xpath expr="//tbody/tr/td[1]" position="before">
            <t t-if="l.animal_qty">
            <td class="text-right">
            <span t-field="l.animal_qty"/>
            </td>
            </t>
            
            </xpath>
     </template>
</data>
</openerp>