Stem characteristics of two forage maize (Zea mays L.) cultivars varying in whole plant digestibility. IV. Changes during the growing season in anatomy and chemical composition in relation to fermentation characteristics of a lower internode

Improving digestibility of forage maize (Zea mays L.) through breeding is important to optimize the efficiency of ruminant's rations. It can partly be achieved by improving the digestibility of stem tissue, a genetically complex and diverse trait changing drastically during the growing season. We tried to gain insight into this trait by analysing the changes during the growing season in the anatomy, chemical composition and fermentation characteristics of a lower internode (internode 7) of two forage maize cultivars differing in whole plant digestibility. These two cultivars, known to differ in digestibility, were grown in the Netherlands for two growing seasons. Cell wall thickness of the sclerenchyma tissue in the rind of internode 7 increased linearly with the temperature sum until reaching final cell wall thickness several days before anthesis. Volens, the less digestible cultivar, had a higher final cell wall thickness than Vitaro, the cultivar with a better digestibility. Chemical analyses included determination of NDF, ADF, ADL, crude protein, sugar content, and ash. Lignin content increased until shortly after anthesis in both cultivars, in both years. Lignin content was higher for Volens than for Vitaro and higher in 2000 than in 1999. Crude protein content decreased from 15–21% in early July to 2–5% in late September with no clear differences between cultivars or years. Fermentation characteristics showed that maximum gas production of cell wall components was highest immediately before anthesis and subsequently decreased. At all stages of development, Vitaro had a higher maximum gas production than Volens. The cultivar differences in digestibility could not be confirmed by differences in rate of cell wall disappearance. Seasonal changes showed an increase in fermentable cell wall material until anthesis; thereafter fermentability decreased. Differences in cell wall thickness and in lignin content reflected the changes in digestibility during the growing season best; the differences between the two contrasting cultivars were best reflected by the differences in cell wall thickness, lignin content and the decline of the potential digestibility in the period before anthesis.