Diversify into cow futures?

I'm holding my fire on the issue of the Speaker until we hear what he says to the Commons today. Meanwhile, comme Le Croydonian, a brief look across the Manche at cow futures.

Even the most urban of French imagine they carry the soil of their native France Profund on their boots, so when bank interest rates on savings are plunging, what is more natural than investing in cows? What they call Holsteins and we call Fresians - the milk factories of the bovine world - can bring a return of 4%-5% a year after tax, it's claimed.

The English language website inviting British investors to join the, er, herd offers the graphic below to illustrate the returns possible. I can see just one tiny flaw ....