Comparison of major carbohydrate fractions and cell wall digestibility in silages made from older and newer maize genotypes grown in the UK

Two populations of UK produced maize silage were studied to compare the dry matter (DM, as an index of plant maturity at harvest), starch and cell wall (as neutral detergent fibre, NDF) contents and the in vivo digestibility of organic matter (OM) and cell walls in maize silages made from early hybrids in the 1970s and 1980s (Population ‘O’) with those made from modern hybrids in the 1990s (Population ‘N’). Relationships between these measurements and DM content were also studied. Mean starch and DM contents were higher (P<0.01) in Population ‘N’ than ‘O’ although NDF contents did not differ (P>0.05). Mean digestible OM content of the DM (DOMD) tended to be higher in Population ‘N’ than ‘O’ but this did not reach significance (P>0.05), although mean NDF digestibility was considerably lower in Population ‘N’ than ‘O’. For both NDF and starch, relationships with DM content derived from the combination of Populations ‘O’ and ‘N’ were non-linear (P<0.001). The second-order regressions were (as g kg−1 DM): Population ‘O’+‘N’ starch=−525+(4.09 DM)−(0.00473 DM2), R2=78.2%Population ‘O’+‘N’ NDF=1255−(4.81 DM)+(0.00694 DM2), R2=63.6% NDF digestibility in vivo was more dependent on NDF content than on DM content giving rise to the possibly that NDF content is a better index of maturity than DM content. NDF digestibility in vivo was very satisfactorily predicted (R2=83.3%, R.S.D.=0.042) by an equation: NDF digestibility coefficient=(DOMD−starch)/(950−starch) (all as g kg−1 DM). Newer varieties of maize have achieved the breeding objective of being earlier maturing with a higher starch content but this may have been at the expense of lower cell wall digestibility with little overall effect on OM digestibility. Plant breeding objectives should include cell wall digestibility in the rumen.