Influence of stage of maturity of grass silages on digestion processes in dairy cows

Because of the introduction of a milk quota system in 1984 and the subsequent decrease of the number of dairy cows with some 25%, an increasing number of farms in the Netherlands has a surplus of grass and grass silage, which makes it interesting to increase the roughage proportion in the diet. However, roughage intake by dairy cows in early lactation is limited and the mechanisms controlling roughage intake are still insufficiently understood. Factors presumably influencing roughage intake are the volume of the feed, the rate of physical size reduction, degradation rate in the rumen, passage rate of undigested particles from the rumen and the removal of fermentation end products.In literature, the results of various experiments are reported in which one or a few of these factors, though not all together in an integrated system, were studied. Therefore, in the experiments described in this thesis, the effect of stage of maturity of grass silages on intake, digestibility, rumen fermentation pattern, rumination activity, passage rate from the rumen, degradation rate of the potentially degradable fractions and composition of the rumen contents was studied in an integrated approach.Four experiments with lactating dairy cows fed grass silages harvested at different growth stages ad libitum and a fixed amount of concentrates, 1 or 7 kg depending on stage of latation, were performed.When grasses matured, the chemical composition changed, the cell wall content (Neutral Detergent Fibre) of the silages and the lignin content of the cell wall fraction increased while crude protein (CP) content decreased. Together with this change in composition, the degradation characteristics changed. The soluble fraction (f s ,%), which is supposed to be fully and rapidly degraded in the rumen decreased, the rumen undegradable fraction (residue after 336 h nylon bag incubation, f R , % ) increased whereas the rate of degradation of the degradable, non-soluble fraction (kd, %h-1) decreased. This resulted in a decrease in digestibility of the silages (Chapter II) . Of the CP fraction of the silages, f s as well as f R increased with an increase in cell wall content of the silages. The soluble fraction of the CP was more closely related to the DM content than to the NDF content of the silages. The f s -CP decreased with an increase in DM content of the silage. Rate of degradation of the CP decreased, as for the other components, with an increase in cell wall content. Thus, the fraction of dietary protein escaping rumen fermentation increased with an increase in cell wall content, mainly because of the higher undegradable fraction. The latter fraction, however, is also not digestible in the intestines. Thus, the fraction of dietary protein escaping rumen fermentation and digestible in the intestines was presumably higher for the low cell wall silages (Chapter V).Milk production (g Fat Corrected Milk/kg Metabolic Body Weight) and the amount of concentrates consumed (g/kg BW) together explained 64% of the variation in silage dry matter (DM) intake. Silage DM intake decreased with an increase in concentrate intake. A significant contribution to the explanation of variation in intake was given by the N/OM ratio in the silage and the f R . The addition of these factors increased the percentage of the variation in silage DM intake explained to 68% (Chapter II).The pH in the rumen declined with an increase in concentrate intake, whereas VFA concentrations increased. Diurnal variations in ammonia concentrations were higer for the high concentrate level, but the average ammonia concentrations were lower for the high concentrate level (Chapter IV).Chewing and rumination time were recorded for six 24 h periods per cow. When rumination time (RT) exceeded 9 h a day, an increase in cell wall content of the silages did not result in a further increase in RT, but in a decrease in intake. RT per kg silage DM ingested increased with an increase in NDF content of the silages, whereas RT per kg silage NDF tended to decrease. Average size of particles in the rumen increased with maturity of the silages, resulting in an increased faecal particle size (Chapter IV).Total rumen contents (kg) and rumen DM contents (kg) increased when the proportion of concentrates in the diet increased, but no significant relation with cell wall content of the silages was found. Passage rate of the fluid from the rumen was not affected by diet composition. Rate of passage of undigested particles from the rumen, calculated from the logarithmic decline in faecal marker excretion, increased with an increase in NDF content of the silages and with an increase in intake level (Chapter III). In these experiments, it was concluded that the marker used to measure particle passage rate, Cr mordanted NDF, was not representative for total rumen contents. Cr-NDF particles with a size of 0.2-1mm do not have to be reduced in size, but it was calculated that not even they could be representative for particles with a size able to leave the rumen. Therefore an experiment was conducted in which three different particle sizes of Cr-NDF (<0.3 mm, 0.6-1.0 mm and 15-25 mm) were used to determine particle passage rate. In this experiment, 3 lactating and 3 non-lactating dairy cows were fed grass silage ad libitum, with 7 or 1 kg of concentrates, respectively. No significant differences between lactating and non-lactating cows were found, but rate of passage decreased with an increase in Cr-NDF particle size. Passage rate of the smallest particles (<0.3 mm) was considerably lower than passage rate of the fluid phase. The size of the particulate marker has a great influence on the calculated passage rates (Chapter IV).Kinetics of rumen particles are considered to be important in intake regulation, but the rate limiting step in reduction in rumen fill was still not identified. In an experiment with four dry rumen regulated cows, rumen contents and rates of disappearance of different morainal fractions were determined (Chapter VII) . The cows were fed ad libitum grass silages, differing in cell wall content. morainal passage rates were calculated from the logarithmic decline in rumen Cr pool, as well as by dividing the intake of undegradable organic matter (iOM) or indigestible acid detergent fibre (iADF) by the mean rumen pool of these fractions. The thus calculated passage rates using the pools of iOM or iADF were much lower than using the Cr-NDF (particle size 0.2-1 mm), but the differences between silages were comparable.Particle size distributions of rumen contents, collected at different times after feeding were determined. The iADF content of particles passing a 1.25 mm, but retained on a 0.071 mm sieve was determined. From the results of this experiment it was concluded that clearance rate of particles with a size between 1.25 mm and 0.071 mm is the rate limiting step in reduction in rumen f ill, which in turn seems to depend on the degree of digestion.In the General Discussion (Chapter VIII) , the results from these experiments were combined with literature data on rumen fill and clearance rate from the rumen. Multiple regression analysis showed that milk yield and concentrate proportion of the diet explained 77% of the variation in rumen fill. Roughage characteristics did not have a significant influence on rumen DM fill. Rate of clearance, however, significantly decreased with an increase in roughage NDF content. Rumen fill and rate of clearance were not significantly correlated.From this study it may be concluded that important factors such as intake, digestibility and milk production all three were negatively influenced by an increase in silage cell wall content. When intake is not limited by the capacity of the rumen, as for the dry cows in the experiment described in chapter VII, intake increases with an increase in silage cell wall content.