Intake of improved and unimproved pastures in two seasons by grazing weanling horses

Abstract

Intake was estimated using the n-alkane technique in 2 groups of 16 grazing weanling horses at Rutherglen during November 1998 (fillies; mean liveweight = 355 kg) and May 1999 (colts; mean liveweight = 266 kg). Weanlings grazed either improved (phalaris, annual ryegrass and subterranean clover; limed and 150 kg/ha superphosphate) or unimproved (annual ryegrass, silver grass, barley grass and subterranean clover; no lime and 125 kg/ha superphosphate) pastures at both high (1.33 horse/ha) and low (0.8 horse/ha) stocking rates. Weanlings were dosed with 500 mg of C₃₂ alkane (via a 300 mL liquid suspension) daily for 7 days prior to, and during, the 4 days of faecal collection. The concentrations of individual alkanes were measured in samples of faeces and herbage. Intake (g OM/kg LW) was then calculated for each horse. A good relationship (r² = 0.99) was observed between the proportions of herbage and faecal odd-chain alkanes in May 1999. However, in November 1998, the relationship was lower (r² = 0.86). Assuming the recovery of odd-chain alkanes does not vary with chain length, this suggests pasture samples collected in November may have been less representative of what the horses were selecting, particularly for horses grazing unimproved pastures at the high stocking rate (r² = 0.79). Intake estimates were similar when C₃₃ or C₃₁ concentrations were used along with the dosed C32, except at the November 1998 sampling for horses grazing unimproved pastures at the high stocking rate. The only significant effect (P<0.05) was the interaction between pasture type and stocking rate at the November 1998 sampling when intake was estimated using the C₃₁/C₃₂ alkane pair.