Management options to decrease phosphorus and sediment losses from irrigated cropland grazed by cattle and sheep

Abstract

In southern New Zealand, grazing of forage crops is common practice to satisfy feed requirements of animals in winter when pasture growth is limited. This practice has been shown to cause soil physical damage and increased loss of surface water contaminants sediment and phosphorus (P) to water bodies. Strategies to mitigate the loss of sediment and P were trialled on a Pallic soil type (Aeric Fragiaquept) in the North Otago Rolling Downlands of New Zealand. All sites were irrigated and measurements were made of losses in overland and sub‐surface flow from intensive cattle or sheep grazed, winter forage crops, and sheep grazed pasture. Two mitigations (restricted grazing of crop to three hours and the application of aluminium sulphate) were assessed for their potential to decrease contaminant loss from cropland. Volumes of surface runoff and loss of total P, filterable reactive P and sediment showed significant differences (P < 0.05) between the control treatments (i.e. no mitigation) with cattle crop (88 mm surface runoff) > sheep crop (67 mm) > sheep pasture (33 mm). The contribution of irrigation water to overland flow water, as a result of saturation‐excess conditions, varied between treatments with more loss under cattle crop (20% of total) compared with sheep crop (15%) and sheep pasture (11%). These differences are probably an effect of soil physical condition and highlight the importance of accurate irrigation scheduling to keep soil moisture below field capacity. Restricted winter grazing and alum application after grazing significantly (P < 0.05) decreased P losses in surface runoff under cattle (from 1.4 to 0.9 kg P/ha) and sheep (from 1.0 to 0.7 kg/P/ha) grazed crop plots by about 30%. In cattle grazed plots, restricted grazing also decreased suspended sediments (SS) by 60%. The use of restricted grazing is suggested as a means of decreasing P and SS loss from grazed winter forage crops. The use of alum shows some promise for decreasing P losses, but requires further work to determine its long‐term effectiveness and use in other soils and management regimes.

Bayesian Network for Point and Diffuse Source Phosphorus Transfer from Dairy Pastures in South Otago, New Zealand

“…napobrassica and kale Brassica oleracea spp . acephala ) has been found to significantly increase P losses in runoff (McDowell and Houlbrooke, 2009) and is another potentially important source of P in New Zealand farm systems. However, only 9% of farmers in the area winter their livestock on‐farm (Kira et al, 2008).…”

“…Phosphorus can originate from point and diffuse sources on a farm. Diffuse sources common to New Zealand dairy farming systems include soil and dung, which are mobilized as dissolved or particulate P (McDowell and Houlbrooke, 2009; McDowell et al, 2003b; Nguyen et al, 1998). Point sources of P are usually clearly identifiable and include cattle lanes and tracks, stock accessing streams, and direct discharges of effluent (Longhurst et al, 2000; McDowell et al, 2005a; Srinivasan and McDowell, 2009; Withers et al, 2009).…”

Evaluation of two management options to improve the water quality of Lake Brunner, New Zealand

“…It is also possible that Al was present, but ineffective, in this grazed pasture soil. Although some work has shown that alum decreases P losses from grazed winter forage crops (McDowell & Houlbrooke 2009) or when incorporated with manure sprayed onto pastures (Moore & Edwards 2007), both of these studies concerned areas with annual rainfall (including irrigation) of about 1100 mm. Additional work is required to determine the right application method and the effectiveness of alum in decreasing P losses from grazed pastures in higher rainfall environments.…”

“…This implies that low watersoluble P fertilisers are safe to apply in areas where runoff is frequent, but no data exist to confirm this within a grazed systems context. To improve soil P retention, some work has indicated that adding Al or Fe oxides or calcium (Ca) to soils may decrease P loss (McDowell & Houlbrooke 2009;Stout et al 1999). However, no field work has been done in grazed pastures.…”

The Use of Alum to Decrease Phosphorus Losses in Runoff from Grassland Soils

“…However, the vast majority of grazed grasslands receive much less rainfall. In another study, alum at 40 kg Al ha −1 was shown to decrease P losses from grazed and irrigated (annual precipitation, 1100 mm) forage crops by about 30% (McDowell and Houlbrooke, 2009). …”