Influence of regrowth age on the nutritive value of dwarf elephant grass hay (Pennisetum purpureum Schum. cv. Mott) consumed by lambs

The change in chemical composition of dwarf elephant grass hay cut at 30, 50 70 and 90 days of regrowth, and its effect on intake, apparent digestibility, ruminal microbial protein synthesis and N balance in lambs (23 ± 2.5 kg live weight), were studied in a double 4 × 4 Latin Square experiment. Diets were chopped hay fed twice a day with a 100-200 g/kg orts. Dry matter (DM) and organic matter (OM), as well as neutral detergent fibre (NDF) and acid detergent lignin (ADL) contents did not differ among regrowth ages, but a linear increase of acid detergent fibre (ADF) (P < 0.01) and non-structural carbohydrates (NSC) (P < 0.05) levels occurred. Ether extract (EE) and total nitrogen (N) contents decreased quadratically from 30 to 70 days, but increased again at 90 days of forage regrowth (P = 0.14 and P = 0.15, respectively). Non-protein N (NPN) accounted for almost all soluble N (SN) contents, which increased quadratically from 30 to 70 days and decreased at 90 days of regrowth (P < 0.05). Intake of DM and OM were not influenced, but NDF intake decreased (P < 0.05) and NSC intake increased linearly (P < 0.01) with the increase of dwarf elephant grass regrowth age. Apparent DM, OM and NDF digestibility, as well as the TOMD, decreased linearly (P < 0.01) with the increase of regrowth age. Intake (P < 0.05), apparent digestibility (P < 0.01), urinary excretion (P < 0.01) and N retention (P < 0.05) were related quadratically with regrowth age. The lowest N intake and the highest N retention were obtained at 70 days of dwarf elephant grass regrowth. Flow of ruminal microbial N into the small intestine, as well as the efficiency of ruminal microbial protein synthesis were not influenced by the dwarf elephant grass regrowth age (P > 0.05). The ADF content was the hay component best related with apparent DM and NDF digestibility. Factors besides regrowth age, such as the climate conditions, may have influenced the chemical composition of dwarf elephant grass. Within regrowth period, days of regrowth was not a good indicator of nutritional value of elephant grass.