The effect of crop maturity on the nutritional value of maize silage for lactating dairy cows 2. Ruminal and post-ruminal digestion

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 can nula were offered four diets in a 4 X 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 sta rch contents. The mean values for volatile-corrected DM (VCDM) and starch c ontent of the four maize silages as given were 221, 277, 308 and 372 g/kg F W and 173, 257, 328 and 382 g/kg VCDM respectively. Grass silage (GS) conta ining 250 g VCDM per kg FW was produced from the primary growth of a perenn ial 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 fora ge treatments were a 3:1 DM ratio of maize silage with GS, designated as T2 3, T28, T33 and T38. Each period lasted 6 weeks with rumen and duodenal sam ples being taken over 3 days in week 4 and faeces being collected in respir ation chambers over 6 days in either week 5 or 6. Milk yield tended to incr ease with advancing maturity (30.5, 31.8, 32.5 and 32.3 kg/day) but individ ual 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 dig estibility in the rumen, post-ruminal tract or total tract. Digestibility o f neutral detergent fibre in the rumen declined with increasing starch cont ent in the maize silage (P < 0.05) but total digestibility was not signific antly affected. Starch intake increased from 3.11 to 5.04 kg/day (P < 0.001 ), duodenal pow from 0.40 to 0.89 kg/day (P < 0.01) and the amounts digeste d in the rumen and post-ruminal tract respectively from 2.72 and 0.34 kg/da y to 4.16 and 0.71 kg/day (P < 0.01). However the only significant effect o n 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 eff ects on nitrogen digestion. The molar proportions of acetic acid and n-capr oic acid decreased and that of n-butyric acid increased with advancing matu rity. 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 fermentati on. Post-ruminal starch digestion doubles but this is due to the increased starch concentration of the silage rather than major changes in digestion a nd the amount is small compared with that likely to result from feeding mai ze grain.