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

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Abstract

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): PopulationO’+‘Nstarch=−525+(4.09DM)−(0.00473DM2),R2=78.2%PopulationO’+‘NNDF=1255−(4.81DM)+(0.00694DM2),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.

Introduction

Maize silage is a major feed resource in the EU and the area grown has increased from about 1 Mha in 1973 to nearly 4 Mha in 1995 (Carpentier, 1996). The UK in particular has seen a rapid increase in the area of maize grown for ensilage from about 14 kha in 1982 to 110 kha in the late 1990s (Mansbridge and Moss, 1996). This increase has been due in large part to availability of earlier maturing and higher yielding genotypes.

As forage maize matures, starch levels increase whilst cell wall content declines (Givens et al., 1995) and it has been shown (Sutton et al., 1999) that this results in a major shift in the ratio of digested starch to digested NDF. Despite these major changes in the carbohydrate composition of the plant with increasing maturity, many studies have also indicated that whole plant apparent digestibility changes little (e.g. Aerts et al., 1984, Brown et al., 1999) although some studies have shown digestibility to decline somewhat (e.g. Bal et al., 1997). Similarly, it is known that maize genotype can substantially affect the quantity and nature of starch (Philippeau et al., 1996) and cell walls (Flachowsky et al., 1993) in forage maize. Digestibility is also affected by genotype, and breeding for high digestibility has been practised in Europe since the early 1980s (Cherney and Cherney, 1994). Work of Deinum (1988) showed that a high proportion of the genotypic variability in forage maize digestibility was due to differences in cell wall digestibility and stressed the need to breed for more digestible cell walls. This, together with the recent conclusions of Tovar-Gómez et al. (1997) that much of the response in animal performance associated with different maize genotypes was due to differences in the extent of cell wall degradability in the rumen, emphasise the importance of a knowledge of cell wall quality.

Most of the data cited above have been obtained from forage maize before ensiling. There are few large scale studies with maize silage and in particular little information from large populations of UK grown maize on many of these important nutritional characteristics and how these may have changed with time. The purpose of the work reported here was to compare, for UK produced maize silage, the starch and cell wall contents and the digestibility of organic matter (OM) and cell walls in maize silages made from early hybrids of the 1970s and 1980s with those made from modern hybrids of the 1990s. Relationships between plant maturity and composition and digestibility were also compared. In addition, the ability to predict cell wall digestibility from more readily available measurements was examined.

Section snippets

Maize silages

Two populations of maize silages were used. The first, Population ‘O’, was 32 samples which represented material produced from older maize genotypes and comprised the combination of the maize silages in Populations ‘F’ and ‘R’ described by Givens et al. (1995). The second, Population ‘N’, represented material produced from newer maize genotypes and comprised 29 samples made from forage harvested from 1992 to 1996. The distribution of harvest year by population is shown in Table 1, together with

Cell wall, starch and dry matter contents

The comparison of the cell wall, starch and DM contents of the two populations of maize silage are shown in Table 2. Mean starch and DM contents were higher (P<0.01) in Population ‘N’ than ‘O’ although NDF contents did not differ (P>0.05). For Population ‘O’, starch and NDF concentrations increased and declined, respectively, with increasing DM content. Both relationships were non-linear (P<0.001) and are in Fig. 1. Second-order regressions describing these relationships were (as g kg−1 DM):

Discussion

It has been recognised for many years that as the maize plant matures, water soluble carbohydrates are translocated from the leaves and stem to the grain for deposition as starch (Daynard et al., 1969) and that the increase in whole plant starch concentration is closely followed by total DM content (Wilkinson and Osbourn, 1975). Thus, the higher DM and starch content of Population ‘N’ (Table 2) is consistent with the increased commercial availability of earlier maturing hybrids in the 1990s and

Conclusions

Newer varieties of maize have achieved the breeding objective of being earlier maturing, thereby having higher DM and starch contents at approximately the same harvest date. However, this may have been at the expense of lower cell wall digestibility with little overall change in OM digestibility. Cell wall digestibility appears to be substantially reduced by increasing plant maturity, is more related to cell wall content of the maize silage than DM content, thereby raising the possibility that

Acknowledgements

Contributions to the funding of this study from the Ministry of Agriculture, Fisheries and Food (MAFF) and the UK Milk Development Council are gratefully acknowledged. We are also grateful to the Centre for Dairy Research (University of Reading) and the Rowett Research Institute (Aberdeen) for helpful collaboration and supply of the forages.

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