using System.Data.Objects.SqlClient;
...
                var z = from a in ctx.ANIMAL
                        where SqlFunctions.StringConvert((decimal?)a.ID_ANIMAL).StartsWith("111")
                        select a;
...

SELECT 
[Extent1].[ID_ANIMAL] AS [ID_ANIMAL], 
[Extent1].[NAME] AS [NAME]
FROM [dbo].[ANIMAL] AS [Extent1]
WHERE STR( CAST( [Extent1].[ID_ANIMAL] AS decimal(19,0))) LIKE N'111%'

USE [DB1]
GO
/****** Object:  UserDefinedFunction [dbo].[GetTotalAmount]    Script Date: 01/11/2013 19:42:14 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
ALTER FUNCTION [dbo].[GetTotalAmount]
(@num as int)
RETURNS nvarchar
AS
BEGIN
return cast(@num as nvarchar)
END

    public partial class DB1Entities1
    {
        [EdmFunction("DB1Model.Store", "ConvertToStr")]
        public static string GetTotalAmount(Nullable<int> num)
        {
            throw new NotImplementedException("Cannot invoke this method");
        }
    }

...

                var z = from a in ctx.ANIMAL
                        where DB1Entities1.GetTotalAmount(a.ID_ANIMAL).StartsWith("1")
                        select a;

SELECT 
[Extent1].[ID_ANIMAL] AS [ID_ANIMAL], 
[Extent1].[NAME] AS [NAME]
FROM [dbo].[ANIMAL] AS [Extent1]
WHERE [dbo].[ConvertToStr]([Extent1].[ID_ANIMAL]) LIKE N'1%'

        <Function Name="ConvertToStringUDF" ReturnType="String">
          <Parameter Name="num" Type="Edm.Int32" />
          <DefiningExpression>
            CAST(num AS Edm.String)
          </DefiningExpression>
        </Function>

    public partial class DB1Entities1
    {
        [EdmFunction("DB1Model", "ConvertToStringUDF")]
        public static string GetTotalAmount(Nullable<int> num)
        {
            throw new NotImplementedException("Cannot invoke this method");
        }
    }

                var z = from a in ctx.ANIMAL
                        where DB1Entities1.GetTotalAmount(a.ID_ANIMAL).StartsWith("1")
                        select a;

SELECT 
[Extent1].[ID_ANIMAL] AS [ID_ANIMAL], 
[Extent1].[NAME] AS [NAME]
FROM [dbo].[ANIMAL] AS [Extent1]
WHERE  CAST( [Extent1].[ID_ANIMAL] AS nvarchar(max)) LIKE N'1%'

 w => SqlFunctions.StringConvert((decimal)w.ID).Trim().StartsWith(term)

 where rtrim(ltrim(STR( CAST( ID AS decimal(19,0))))) LIKE N'808%'

  w => SqlFunctions.StringConvert((decimal)w.ID).Trim().StartsWith(term)

 w => SqlFunctions.StringConvert((decimal)w.ID).Trim()Trim().StartsWith(term)

или

 w => SqlFunctions.StringConvert((decimal)w.ID, 10, 5).Trim().StartsWith(term)