The effect of crop maturity on the nutritional value of maize silage for lactating dairy cows 1. Energy and nitrogen utilization

Four multiparous Holstein-Friesian cows fitted with simple cannulas in the proximal duodenum and the rumen were offered four diets in a 4 X 4 Latin-square design to evaluate the effect on energy and nitrogen balance of crop maturity of maize when offered as ensiled food with grass silage and a concentrate. Forage maize (cv. Hudson) tons ensiled at target dry matter (DM) contents of 230, 280, 330 and 380 g per kg fresh weight (FW). The mean values for volatile corrected DM (VCDM) and starch content of the maize for ages as given were 226, 278, 319 and 357 g/kg FW and 180, 263, 327 and 401 g/kg VCDM respectively. Grass silage (GS) containing 247 g VCDM per kg FW was produced from the primary growth of a perennial ryegrass sword. The diets comprised 8.7 kg DM concentrate per day with one of four forage treatments offered ad libitum, in a 3 : 1 DIM ratio of maize silage with GS, designated T23, T28, T33 and T38. Each period was of 6 weeks with energy and nitrogen balances conducted zn respiration chambers over 6 days in either week 5 or 6. There were no significant effects of maturity on DM intake. Changes in milk yield and composition were not significant but milk protein yield increased significantly (P < 0.05) with maize maturity up to T33 and was linearly related (P < 0.05) to changes in maize silage starch and neutral- and acid-detergent fibre (NDF, ADF) content. Total starch intake increased significantly (P < 0.01) with maturity but apparent digestibility of starch was significantly (P < 0.05) reduced only with the most mature maize silage (T38). NDF and ADF intake and amounts digested were not significantly different despite a numerical dl:cline with stage of maturity Total nitrogen intake and apparent digestibility were not significantly different although there were significant differences(P < 0.05) in the amount of nitrogen excreted as urine, which was greatest on T23 and least on T33, and milk which was least on T23 and greatest on T33. Mean gross energy (GE) intake increased by 17 MJ/day from T23 to T33 but the differences were non-significant. Faecal energy output on T23 was significantly lower (P < 0.05) than the other treatments whilst urine energy on T23 was significantly higher (P < 0.05) compared with T33. Methane energy losses were not significantly different. There were no significant differences between treatments in either GE digestibility or metabolizability, digestible or metabolizable energy (ME) intakes or in the partition of ME to heat, tissue or milk. The calculated ME concentrations of the maize silages were not significantly different with an overall mean of 12.3 MJ/kg VCDM estimated at maintenance intake, or 0.63 MJ/MJ GE. Changes in maize silage composition resulted in a doubling of the ratio of digested starch to digested NDF (0.66, 0.94, 1.10 and 1.21) for treatments T23 to T38 respectively Despite this large change in digested nutrients no differences in the efficiency of energy utilization were detected.