Using formnovalidate in ASP.Net TextBox

Hello!

I have a problem in my TextBox. I want to have an option TextBox in my program like txtSpecies is optional and I put formnovalidate but it required me to input the data.

Can anyone have an idea?

<asp:TextBox ID="txtSpecies2" runat="server" CssClass="form-control" 
    placeholder="Enter Species1" required="required" Width="183px"/>
<asp:TextBox ID="txtSpecies2" runat="server" CssClass="form-control"
    placeholder="Enter Species2" required="required" Width="183px" formnovalidate/>