Lax leaf maize: cell wall composition and nutritional value

Forage nutritive value, which comprises traits such as digestibility, fibre, lignin and protein content, is an important criterion for maize (Zea mays L) harvested as silage. Lines with a characteristic phenotype ('lax leaf') could be useful sources of genes for improved nutritive value in maize. A study was conducted to characterise the cell wall composition of the lax leaf line. Lax leaf inbreds and inbreds representing 'normal' maize were evaluated for cell wall neutral sugars, uronic acids, Klason lignin and phenolic acids in five tissues from the ear node and the internode above it. Acid detergent fibre (ADF) and neutral detergent fibre (NDF) and 48 h in vitro true digestibility (IVTD) were predicted using near-infrared reflectance spectrophotometry (NIRS) calibrated with a subset of the scanned samples. Lax leaf inbred tissues had lower levels of ADF, NDF, lignin and xylose and were more digestible than tissues from the inbreds representing 'normal' maize. It was not known whether the lax leaf phenotype resulted from alterations in nutritive value traits or whether laxness and nutritive value traits are independent from one another. A second study was conducted to determine the nature of genetic control of the lax leaf character and to determine the genotypic relation between the lax leaf character and nutritive value. A recombinant inbred mapping population was developed from a cross between the lax leaf line and an inbred line with stiff upright leaves. Whole-plant samples from each recombinant inbred line were evaluated for ADF, NDF, acid detergent lignin (ADL) and IVTD of dry matter using NIRS. Laxness, measured by number of broken leaves, was associated with lower nutritive value in this population (genetic correlations 0.16-0.34), which was contrary to expectation. Amplified restriction fragment length polymorphism (AFLP) and simple sequence repeat (SSR) markers were used to identify linkage groups associated with the lax leaf character, digestibility and fibre content. Several linkage groups were associated with both the lax leaf character and nutritive value. Where these characters were associated with the same linkage group, the lax leaf parent allele was associated with greater laxness but reduced nutritive value. The lax leaf parent allele was associated with increased nutritive value in linkage groups unassociated with the lax leaf character. While the lax leaf line may be a good source for alleles for improved nutritive value, selection for laxness will not likely be accompanied by improvement in forage quality.