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-sq uare design to evaluate the effect on energy and nitrogen balance of crop m aturity of maize when offered as ensiled food with grass silage and a conce ntrate. Forage maize (cv. Hudson) tons ensiled at target dry matter (DM) co ntents 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 pr oduced from the primary growth of a perennial ryegrass sword. The diets com prised 8.7 kg DM concentrate per day with one of four forage treatments off ered ad libitum, in a 3 : 1 DIM ratio of maize silage with GS, designated T 23, T28, T33 and T38. Each period was of 6 weeks with energy and nitrogen b alances 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 mi lk yield and composition were not significant but milk protein yield increa sed 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 signific antly (P < 0.01) with maturity but apparent digestibility of starch was sig nificantly (P < 0.05) reduced only with the most mature maize silage (T38). NDF and ADF intake and amounts digested were not significantly different d espite a numerical dl:cline with stage of maturity Total nitrogen intake an d apparent digestibility were not significantly different although there we re 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 ene rgy output on T23 was significantly lower (P < 0.05) than the other treatme nts whilst urine energy on T23 was significantly higher (P < 0.05) compared with T33. Methane energy losses were not significantly different. There we re 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 concentrati ons of the maize silages were not significantly different with an overall m ean of 12.3 MJ/kg VCDM estimated at maintenance intake, or 0.63 MJ/MJ GE. C hanges in maize silage composition resulted in a doubling of the ratio of d igested starch to digested NDF (0.66, 0.94, 1.10 and 1.21) for treatments T 23 to T38 respectively Despite this large change in digested nutrients no d ifferences in the efficiency of energy utilization were detected.