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 wal l las neutral detergent fibre, NDF) contents and the in vivo digestibility of organic matter (OM) and cell walls in maize silages made from early hybr ids in the 1970s and 1980s (Population 'O') with those made from modern hyb rids in the 1990s (Population 'N'). Relationships between these measurement s 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 highe r 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 fr om the combination of Populations 'O' and 'N' were nonlinear (P < 0.001). T he second-order regressions were (as g kg(-1) DM): Population 'O' + 'N' starch = -525 + (4.09 DM) - (0.00473 DM2), R-2 = 78.2% Population 'O' + 'N' NDF = 1255 - (4.81 DM) + (0.00694 DM2), R-2 = 63.6% NDF digestibility in vivo was more dependent on NDF content than on DM cont ent giving rise to the possibly that NDF content is a better index of matur ity than DM content. NDF digestibility in vivo was very satisfactorily pred icted (R-2 = 83.3%, R.S.D. = 0.042) by an equation: NDF digestibility coeff icient = (DOMD - starch)/(950 - starch) (all as g kg(-1) DM). Newer varieti es 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. Pl ant breeding objectives should include cell wall digestibility in the rumen . (C) 2001 Elsevier Science B.V. All rights reserved.