What was meant by ‘ferd corn’ or ‘third corn’?

the ferd corn (literally ‘the fourth corn’), and the third corn were estimates of the value of a measure of grain sown, that is, they multiplied the value of the amount sown to take account of the eventual yield. 

The example below comes from the inventory of a 17th century testament: viz. in the borneyaird the sawing of Ten f[i]r[lottis] aittis estimat
to the thrid corne pryce of the boll w[i]t[h] the fodder
thrie li[bri] vj s[olidi] viij d[enarii] Inde xxv li[bri] Item the sawing of
thrie f[i]r[lottis] beir estimat to the ferd corne pryce of the
boll with the fodder v li[bri] Inde xv li[bri] 

That is:
in the barnyard the sowing of 10 firlots oats estimated to the third corn ( X 3 ) = 30 firlots (that is, 7.5 bolls) @ £3 6 shillings and 4 pence per boll (including fodder) = £25
and the sowing of 3 firlots barley estimated to the ferd corn ( X 4 ) = 12 firlots (that is, 3 bolls) @ £5 per boll (including fodder) = £15