Phosphorus Losses from Simulated Dairy Mangement Intensive Grazing Forage System

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Virginia Tech


Dairy producers across the country are evaluating the effectiveness of management intensive grazing (MIG) systems as a means of reducing the economic pressures of confinement feeding and manure handling. Systems using MIG have been promoted as an environmentally safer way of managing nutrient balance on Center. However, little research has been conducted to evaluate how these systems affect phosphorus (P) loss from the Center through runoff and forage removal. The goal of this study was to investigate the effects of forage type and manure density on P levels in runoff from release plots that simulated a MIG dairy system.

Two forage treatments were planted on runoff release plots and applied with three manure density treatments and four replications of each combination for a total of 24 plots. Orchardgrass (Dactylis glomerata) and a broad-leafed forage treatment consisting of buckhorn plantain (Plantago lanceolata), red clover (Trifolium pratense), and alfalfa (Medicago sativa) were planted on the runoff release plots in early spring. Manure densities with no manure (control), a density simulating a low stocking density (low), and a density simulating high stocking density (high) were applied to plots during grazing simulations. Six rainfall simulations were conducted over the season simulating grazing conditions when forage was removed and re-growth conditions when forage had reached its target re-growth height before re-grazing occurred. Thirty minutes of runoff was collected from the runoff release plots to determine first flush phosphorus losses from the various treatments.

Mass losses of total phosphorus (TP) for manure treatments in series 1 were significant due to plot installation disturbance. No significant effect for ortho-phosphorus (ortho-P), particulate phosphorus (PP) or TP concentration or mass losses were observed for other simulation series. Significantly higher mass losses were observed in series 1 for both forage treatments, again due to installation disturbance. Seasonal mass losses for TP were significantly lower for the orchardgrass treatment.

Forage mass removal over the season increased as the stands established. Orchardgrass treatments appeared to control P losses earlier in the season, while broadleaf treatments observed a steady increase in ortho-P losses up to series 5. Both mass and concentration losses of TP were controlled earlier in the season by orchardgrass, while PP showed no significant mass or concentration loss effects.



Phosphorus, pasture management, nonpoint source pollution, management intensive grazing, dairy grazing, pasture forages