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

Abstract

Changes in pasture metabolisable energy (ME), crude protein (CP), neutral detergent fibre (NDF), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S), sodium (Na) and chlorine (Cl) were measured at each grazing, 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 quality parameters using measures of pre- and post-grazing masses. Changes in botanical composition and pasture allowance were also measured.

The nutritive characteristics of pasture on both farms followed similar trends. Metabolisable energy and CP were highest in winter and early spring with values of over 11 MJ/kg DM and 250 g/kg DM respectively. The lowest concentrations of ME and CP in pasture on farm A (9.4 MJ/kg DM; 128 g/kg DM) were 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 differentials 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 values generally indicating a positive selection differential (P 1.12, 1.15; Ca 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/cow.day and there was no relationship between selection differentials for all nutritive characteristics and pasture allowance.

The results obtained in this study indicated that although the ME of pasture consumed throughout the year should be adequate to meet production of 30 L/cow.day in early lactation with minimal liveweight loss, it was associated with high levels of CP and potentially low concentrations of NDF in relation to cow requirements. Results therefore indicate a possible need for supplementing pasture diets in winter and spring to balance the diet for CP and NDF. In addition, it is likely that both Ca and Mg intake from pasture may be limiting in early lactation and therefore additional supplements of both minerals may be required during this period.