EFFECTS OF AGE AND SEASON ON GROWTH AND NUTRITIVE-VALUE OF RHODES GRASS (CHLORIS-GAYANA CV KUNTH)

To study the effects of age and season on the composition and digestib ility of Rhodes grass, 40 cuts replicated four times were harvested fr om one primary and nine secondary growths. Starting dates for growths were 3 December (Day 1) until 21 April (Day 140) and cuts were made su ch that 4-, 6-, 8- and 10-week-old material was obtained from ten star ting dates between Day 1 and Day 140. Rainfall was fairly well distrib uted between mid-December and the end of April. Maximum temperature (M AXT) varied slightly around 24 degrees C, whereas minimum temperature (MINT) dropped from about 13 to 6 degrees C between early May and the end of June. Solar radiation varied considerably and increased from Ja nuary and onwards, Environmental effects on growth rate and plant comp osition were weak and inconsistent, Only the effect of MAXT was signif icantly (negatively) associated with crude protein (CP) content in the 6-10 week forages and positively associated with neutral detergent fi bre (NDF) content. The number of significant effects obtained from the regression of environmental factors upon plant composition increased with forage maturity, It is likely that differences were not fully exp ressed in the less mature grass. The results showed that growth rate, CP, NDF, lignin (LGN), hemicellulose/cellulose (HC/CEL) and in vitro o rganic matter digestibility (IVOMD) were all significantly (P < 0.01) affected by age and day of season, except for the effect of season on LGN. The interactions between age and day of season were significant ( P < 0.01) for growth rate, CP, NDF and IVOMD. Growth rate and NDF incr eased with age from 48 to 63 kg DM ha(-1) day(-1) and from 64 to 70%, respectively, between 4 and 10 weeks of age. Crude protein and IVOMD d ecreased from 17 to 9% and from 81 to 68%, respectively. The growth ra te of the forages declined with day of season by -0.38 kg DM ha(-1) da y(-1) probably as an effect of decreasing temperature and rainfall tow ards the end of the reason. The regression of day of season was signif icant (P < 0.05) for IVOMD but the magnitude was marginal. Leaves were higher in CP, HC/CEL and IVOMD and lower in NDF and LGN than stems. ( C) 1997 Elsevier Science B.V.