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 anthesi

Stem characteristics of two forage maize (Zea mays L.) cultivars varying in whole plant digestibility. II. Relation between in vitro rumen fermentation characteristics and anatomical and chemical features within a single internode

Internode 7 of the stem of two forage maize (Zea mays L.) cultivars was studied anatomically and chemically at anthesis and subjected to fermentation tests in rumen fluid, using an automated gas production system. For anatomical studies internode 7 was sectioned at the top, middle and base. For chemical analyses and fermentation studies, the internode was divided into five sections. The number of sclerenchyma layers increased from top to base of the internode, whereas cell wall thickness of the sclerenchyma decreased from top to base. The highest and lowest sections of the internode had a higher sugar content and lower levels of neutral detergent fibre, acid detergent fibre, and acid detergent lignin, and slightly higher levels of crude protein than the middle three sections. Also their hemicellulose and cellulose contents were different but the differences with the middle three sections were small. Maximum gas production from fermentation in rumen fluid was higher for cell wall material from the highest and lowest than from the middle three sections. This was mainly due to differences in chemical composition: the highest and lowest section had a higher lignin and a relatively high hemicellulose content compared with the middle three sections. Digestibility of the cultivar with the higher whole plant digestibility, Vitaro, exceeded that of the cultivar Volens for each of the sections examined within the selected internode. Secondary cell wall disappearance was only weakly correlated with lignin content or gas production. This suggests that other factors are important in the fermentation process. These may include anatomical factors influencing the accessibility of the tissues to micro-organisms and chemical and/or physical factors such as lignin composition, lignin localization within the cell wall and nature and extent of cross-linking lignin to other cell wall components

Stem characteristics of two forage maize (Zea mays L.) cultivars varying in whole plant digestibility. I. Relevant morphological parameters

The morphology and rumen fermentation kinetics of the maize cultivars (Zea mays L.) Vitaro and Volens were investigated in detail throughout their growing period as a first step towards understanding the relation between plant characteristics and cell wall fermentability of forage maize. Vitaro is known to have a 9% higher whole plant organic matter digestibility than Volens. Leaf and internode development, fresh (FW) and dry weight (DW) per plant and dry matter content (DMC) of leaves, internodes and developing ears, as well as rumen fermentation characteristics of the stem, were monitored during two seasons. Vitaro plants had a larger final leaf area than Volens plants but their number of leaves (and internodes) was the same. Fully developed Vitaro internodes were shorter and thicker than Volens internodes, resulting in a shorter plant for Vitaro. After anthesis, FW and DW of individual internodes did not vary significantly throughout the growing period. Whole plant FW increased sharply after anthesis, which was associated with the development of the main ear. In both cultivars, DMC of the whole plant more or less doubled between anthesis and harvesting. Vitaro had a higher DW per plant than Volens, but not a larger ear proportion. In rumen fermentation tests on whole stem samples, using the gas production technique, gas production after 72 hours and Tilley & Terry digestibility were significantly higher for Vitaro than for Volens, indicating a higher total degradability for Vitaro. It was concluded that the two cultivars form a suitable model system for studying the causes of differences in rumen fermentation between maize roughages

Stem characteristics of two forage maize (Zea mays L.) cultivars varying in whole plant digestibility. III. Intra-stem variability in anatomy, chemical composition and in vitro rumen fermentation

The internodes of forage maize (Zea mays L.) stems were studied at anthesis for variation in anatomy and chemical composition in relation to digestibility. The study was carried out with a short (Vitaro) and a tall (Volens) cultivar differing in whole-plant digestibility, both of which were grown in the field in the growing seasons of 1999 and 2000. Internode diameter increased from the top to the base of the stem and Vitaro had shorter and thicker internodes than Volens. The cell walls of the sclerenchyma tissue in the rind were thicker and the numbers of sclerenchyma layers around vascular bundles in the rind higher in lower than in upper internodes. The neutral detergent fibre content (NDF%) of the internodes increased from the top to the base of the stems of both cultivars, but was very high for the peduncle. NDF% was lower for Vitaro than for Volens in all internodes. The sugar content of the dry matter was highest for Internode 12, i.e., the internode near the position of the ear, and was very low for the peduncle. Vitaro always had a higher sugar content than Volens. When subjected to fermentation tests with rumen fluid in an automated gas production system, gas production values after 3, 20 or 72 h of incubation were higher for internodes from the top than for internodes from the base of the stem, and were lower for the peduncle than for Internode 14. The values were consistently higher for Vitaro than for Volens internodes; in general, this difference was most apparent for Internode 10. The differences in gas production amongst internodes and between cultivars were in line with differences expressed by in vitro digestibility measurements. Fermentation results of cross sections suggest that the cell walls in lower internodes disappeared faster and to a greater extent than the cell walls in upper internodes, except for Volens in 1999, and with the exception of the peduncle. The rate of cell wall disappearance was higher for Volens than for Vitaro, but ultimately similar amounts of cell wall material disappeare