Sampling strategy for estimating dairy-pasture quality
G.P. Cosgrove, K. Betteridge, V.J. Thomas and D.C. Corson
AgResearch, Private Bag 11008, Palmerston North
Proceedings of the New Zealand Society of Animal Production. 1998,
58: 25-28
Rapid determination of pasture quality using near infrared
reflectance spectroscopy (NIRS) makes it feasible for farmers to
incorporate forage composition in feed budgeting and feed
supplementation decisions for grazing animals. In grazed pastures,
heterogeneity in the distribution of the mass and quality, necessitates
a sampling strategy to accurately predict the diet. A single paddock was
systematically sampled by selecting 60 sites according to a
pre-determined grid. At 12 of these sites, 4 separate but adjacent
samples of 150 x 150 mm were taken from within a 300 x 300 mm quadrat.
To characterise vertical heterogeneity, the herbage at each of the 60
sites was cut sequentially into 4 equal strata, the depth of each being
one-quarter of the canopy surface height at that site. These samples
were used to derive a statistically-based 3-dimensional characterisation
of variability of herbage mass and of several parameters of herbage
quality determined using NIRS. This description was used to calculate
the number of samples necessary to estimate parameters to arbitary,
pre-determined levels of accuracy of ± 50 kg DM/ha for herbage mass in
each stratum, ± 50 g/kg protein (PROT), acid detergent and neutral
detergent fibre (ADF; NDF), ± 20 g/kg for soluble carbohydrate (CARB), ±
25 g/kg for in-vitro dry matter digestibility (DIG) and ± 0.5 MJ/kg for
metabolizable energy (ME). A minimum of six samples bulked together is
necessary to estimate PROT, CARB, NDF and ADF to the specified level of
accuracy, and this number was similar for each stratum. Twelve samples
are required to estimate ME and DIG for each of the 3 upper strata, but
in the lowest stratum with greater variability, 48 and 19 samples would
be required for ME and DIG, respectively, to reach the same accuracy. A
sampling strategy should take the suggested number of samples by
systematically covering the paddock area. Uncertainty in the depth of
sampling because of the necessity to sample before grazing has little
effect on the number of samples required or on the accuracy of
estimates, providing the lowest stratum is not penetrated during either
sampling or grazing.
Keywords: NZSAPAB;
dairy-pasture quality; sampling; spatial variability; diet quality.
Last Updated 12-09-1998
