Changes in the botanical composition and nutritive characteristics of pasture, and nutrient selection by dairy cows grazing rainfed pastures in western Victoria

Changes in pasture metabolisable energy (ME), crude protein (CP), neutral d etergent fibre (NDF), phosphorus (P), potassium (K), calcium (Ca), magnesiu m (Mg), sulfur (S), sodium (Na) and chlorine (Cl) were measured at each gra zing, in 4 paddocks on 2 farms (farm A and farm B) in western Victoria from May 1995 to March 1997. Selection differentials were calculated from quali ty parameters using measures of pre- and post-grazing masses. Changes in bo tanical composition and pasture allowance were also measured. The nutritive characteristics of pasture on both farms followed similar tre nds. Metabolisable energy and CP were highest in winter and early spring wi th values of over 11 MJ/kg DM and 250 g/kg DM respectively. The lowest conc entrations of ME and CP in pasture on farm A (9.4 MJ/kg DM; 128 g/kg DM) we re observed in April and February respectively. On farm B the corresponding lowest values (8.4 MJ/kg DM; 100 g/kg DM) were in March. In contrast, NDF values on farm A were lowest in June (466 g/kg DM) and on farm B (436 g/kg DM) in May, with highest values in February (648 g/kg DM) and March (692 g/ kg DM) respectively. Concentrations of P, K, S and Cl in pasture followed a similar pattern to that of ME with highest values in winter and lowest in summer, while changes in Ca concentrations related more closely to changes in NDF. The concentrations of Mg and Na were highest in autumn and spring, respectively, with lowest values in late spring and summer. Selection differentials indicated that the ME of pasture consumed was 4-22% higher than the pasture on offer. For CP the range of selection differenti als was always greater than 1, but varied widely (1.08-1.83). The selection differentials for NDF were always negative, with an average value of 0.84. Selection differentials for minerals varied little over the year, with val ues generally indicating a positive selection differential (P 1.12, 1.15; C a 1.16, 1.16; Mg 1.15, 1.18; K 1.23, 1.27; Na 1.05, 1.07; Cl 1.16, 1.14; S 1.27, 1.28) for farms A and B. Pasture allowances ranged from 10-60 kg DM/c ow.day and there was no relationship between selection differentials for al l nutritive characteristics and pasture allowance. The results obtained in this study indicated that although the ME of pastur e consumed throughout the year should be adequate to meet production of 30 L/cow.day in early lactation with minimal liveweight loss, it was associate d with high levels of CP and potentially low concentrations of NDF in relat ion to cow requirements. Results therefore indicate a possible need for sup plementing pasture diets in winter and spring to balance the diet for CP an d NDF. In addition, it is likely that both Ca and Mg intake from pasture ma y be limiting in early lactation and therefore additional supplements of bo th minerals may be required during this period.