Published online by Cambridge University Press: 18 August 2016
To evaluate the effect of crop maturity on digestion of maize silage in the rumen and post-ruminal digestive tract, four multiparous Holstein-Friesian cows fitted with a simple cannula in the proximal duodenum and a rumen cannula were offered four diets in a 4 ✕ 4 Latin-square design. Forage maize (cv. Hudson) was harvested and ensiled at target dry matter (DM) contents of 230, 280, 330 and 380 g per kg fresh weight (FW) to provide a range of starch contents. The mean values for volatile-corrected DM (VCDM) and starch content of the four maize silages as given were 221, 277, 308 and 372 g/kg FW and 173, 257, 328 and 382 g/kg VCDM respectively. Grass silage (GS) containing 250 g VCDM per kg FW was produced from the primary growth of a perennial ryegrass sward. The diets were 8·7 kg DM per day of a dairy concentrate supplement with one of four forage treatments offered ad libitum. The forage treatments were a 3 : 1 DM ratio of maize silage with GS, designated as T23, T28, T33 and T38. Each period lasted 6 weeks with rumen and duodenal samples being taken over 3 days in week 4 and faeces being collected in respiration chambers over 6 days in either week 5 or 6. Milk yield tended to increase with advancing maturity (30·5, 31·8, 32·5 and 32·3 kg/day) but individual treatment differences were not significant. DM intake increased from 19·62 to 21·30 kg/day (P < 0·05) but there were no significant effects on digestibility in the rumen, post-ruminal tract or total tract. Digestibility of neutral detergent fibre in the rumen declined with increasing starch content in the maize silage (P < 0·05) but total digestibility was not significantly affected. Starch intake increased from 3·11 to 5·04 kg/day (P < 0·001), duodenal flow from 0·40 to 0·89 kg/day (P < 0·01) and the amounts digested in the rumen and post-ruminal tract respectively from 2·72 and 0·34 kg/day to 4·16 and 0·71 kg/ day (P < 0·01). However the only significant effect on starch digestibility was a small fall in total digestibility from a mean of 0·981 for T23, T28 and T33 to 0·966 for T38. There were no treatment effects on nitrogen digestion. The molar proportions of acetic acid and n-caproic acid decreased and that of n-butyric acid increased with advancing maturity. It is concluded that the changes in composition of maize silage with increasing maturity result in large increases in the contribution of starch to DM digested in the rumen but only small differences in rumen fermentation. Post-ruminal starch digestion doubles but this is due to the increased starch concentration of the silage rather than major changes in digestion and the amount is small compared with that likely to result from feeding maize grain.