Abstract
The aim was to evaluate the prediction accuracy of pasture dry matter intake (PDMI) and milk yield (MY) predicted by the GrazeIn model using a database representing 124 PDMI measurements at paddock level and 2232 MY measurements at cow level. External validation of the model was conducted using data collected from a trial carried out with Holstein-Friesian cows (n=72) while grazed 28 paddocks and were managed in a 2×2 factorial design by considering two calving dates (CD), with different number of days in milk (DIM), early (E, 29 DIM) vs. middle (M, 167 DIM), and two stocking rates (SR), medium (M, 3.9 cows ha-1) vs. high (H, 4.8 cows ha-1), under a rotational grazing system. Cows were randomly assigned to four grazing scenarios (EM, EH, MM and MH). The mean observed PDMI of the total database was 14.2 kg DM cow-1 day-1 while GrazeIn predicted a mean PDMI for the database of 13.8 kg DM cow-1 day-1. The mean bias was −0.4 kg DM cow-1 day-1. GrazeIn predicted PDMI for the total database with a relative prediction error (RPE) of 10.0% at paddock level. The mean observed MY of the database was 23.2 kg cow-1 day-1 while GrazeIn predicted a MY for the database of 23.1 kg cow-1 day-1. The mean bias was –0.1 kg cow-1 day-1. GrazeIn predicted MY for the total database with a mean RPE of 17.3% at cow level. For the scenarios investigated, GrazeIn predicted PDMI and MY with a low level of error which made it a suitable tool for decision support systems.
Highlights
Galician (NW Spain) economy is highly oriented to the primary sector
There was a large range in the cow and pasture characteristics, and grazing management input variables in the CIAM database used to perform the external validation of the GrazeIn model (Table 1)
The pasture offered ranged in DM (184-200 g kg-1), crude protein (CP) (127-156 g kg-1 DM), acid detergent fibre (ADF) (287-310 g kg-1 DM), neutral detergent fibre (NDF) (515-537 g kg-1 DM), water soluble carbohydrates (WSC) (154-165 g kg-1 DM) and in vitro organic matter digestibility (IVOMD) (71.7-77.6%)
Summary
61% of the gross value added (GVA) in the region comes from animal production. Pasture-based milk production systems have been decreasing considerably over the last 30 years in other European regions (Bourgeois, 2002) as Galicia, where climatic conditions are adequate for pasture growth (Mayne & Peyraud, 1996) all year around, and the number of cows which are kept indoors for all or part of. Due to an increased world demand for dairy products and high volatility in the price of feedstuffs for animal nutrition is nowadays envisaged that a larger proportion of the milk produced in humid areas might come, after the abolition of EU milk quotas in 2015, from grazing dairy systems as pasture is the cheapest source of nutrients available for feeding ruminants (Dillon, 2006)
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