The effect of supplementary maize silage on rumen function in dairy cows grazing high quality pasture : A thesis submitted in partial fulfilment of the requirements for the Degree of Bachelor of Agricultural Science with Honours at Lincoln University

Abstract

Maize (corn) silage is increasingly being used as a supplementary feed within the New Zealand dairy industry. Supplementing a high quality pasture diet with 4 kg DM/day of maize silage altered the diurnal pattern of rumen pH, with the pH of maize supplemented cows being significantly higher than that of pasture-fed (control) animals for several intervals of the day. Consequently, the diurnal rumen pH curve was flatter as a result. This increase in mean rumen pH with maize silage supplementation is in contrast to previous literature involving total mixed ration systems. However, while animals supplemented with the maize silage had an increased mean rumen pH, there was also an increase in the mean number of daily bouts below pH 5.5 (2.4 v. 1.5), though the mean duration of each bout did not increase. This was offset by a greater time above pH 6.0 each day, which was responsible for raising the overall mean diurnal pH in maize supplemented cows. Maize silage supplementation also significantly (P<0.05) reduced the ruminal concentration of ammonia (17.0 mg/L v. 22.8 mg/L) and propionic acid (20.9 mM v. 23.8 mM), compared with that of the pasture fed group. Consequently, the acetic to propionic (A: P) ratio was significantly (P<0.05) higher in the maize supplemented group (4.3 v. 3.9). This decrease in propionic acid concentration is in contrast with other reports of feeding increased starch-based rations. However, there were no negative effects of 'slug' feeding (high intake rate over a short period of time) on rumen function, as have been reported in previous trials, particularly in total mixed ration systems overseas. Maize silage supplementation appeared to reduce pasture intake as estimated by the C32 alkane method, due to total daily DMI being unchanged between treatments (14.12 kg DM/d v. 14.73 kg DM/d). Mean faecal (6.48 v. 6.53) and urine pH (8.17 v. 8.16) were not significantly different between treatment groups, suggesting there was insignificant starch in the ,ration to alter hindgut fermentation patterns. The rumen microbial community was altered by maize silage supplementation, as assessed by Denaturant Gradient Gel Electrophoresis of 16S RNA. The overall diversity of the rumen microbial community was reduced, which is in contrast with previous reports in the international literature. In conclusion, maize silage supplementation to high intake dairy cows grazing high quality pasture did not produce any detrimental effects in rumen function, but did appear to cause substitution of pasture intake.