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USDA (2003) estimated the amount of manure produced from animal distribution numbers from the 1997 U.S. Census of Agriculture, using standard values of manure production and nutrient concentration for each animal type. Estimates of excess N and P were calculated based on crop N and P removal and the assumption that all suitable crop and pasture land was available for manure application (Figure 43 and Figure 44). Most areas with CAFOs have some excess N (Figure 43) and P (Figure 44). These distributions demonstrate that within the MARB, regional excesses were similar for N and P.
Targeting Remedial Strategies Within the MARB
The importance of targeting nutrient management within a watershed is shown by several MARB studies. In the early 1980’s conservation practices were installed on about 50% of the Little Washita River watershed (54,000 ha or 133,000 ac) in central Oklahoma. Practices included construction of flood control impoundments, eroding gully treatment, and conservation tillage (Sharpley and Smith, 1994; Sharpley et al., 1996). Although conservation measures decreased N and P export 5 to 13 fold, there was no effect on P concentration in flow at the outlet of the main Little Washita River watershed. Thus, a lack of effective targeting of nutrient management and control of major sources of nutrient export contributed to field or subwatershed scale responses not being translated to reductions in nutrient export from the main Little Washita River watershed.
Manure application timing and method relative to rainfall influences the concentration of N and P in runoff (Dampney et al., 2000; Sims and Kleinman, 2005). For example, several studies have shown a decrease in N and P loss with an increase in the length of time between manure application and surface runoff (Djodjic et al., 2000; Edwards and Daniel, 1993a; Sharpley, 1997; Westerman et al., 1983). This decrease can be attributed to the reaction of added P with soil and dilution of applied P by infiltrating water from rainfall that did not cause surface runoff.
The incorporation of manure into the soil profile either by tillage or subsurface placement decreases the potential for P loss in surface runoff. Rapid incorporation of manure also reduces NH3 volatilization and potential loss in runoff as well as improving the N:P ratio for crop growth. Mueller et al. (1984) showed that incorporation of dairy manure by chisel plowing reduced total P loss in runoff from corn 20-fold, compared to no-till areas receiving surface applications. In fact, P loss in runoff was decreased by a lower concentration of P at the soil surface and a reduction in runoff with incorporation of manure (Mueller et al., 1984; Pote et al., 1996). As with fertilizer application methods, other factors are important in selecting or recommending the most appropriate application method. Equipment availability, whether the soil is sufficiently free of rocks to allow subsurface application, labor requirements, product availability, and availability of operating capital all affect the application method decision.
Crop selected to receive manure application
Manure has traditionally been applied for corn or other grass production. However, corn acreage to which manure is applied has not expanded proportionally to animal operation expansions; thus the risks increase for applying manure in excess of the amount necessary to meet crop nutrient requirements (Schmitt et al., 1996; Dou et al., 1998). One solution to minimize these risks, and the subsequent potential risk of NO3 leaching to ground water, is to select alternative crops to receive manure applications. Although legumes are not usually considered for manure application, soybean can annually remove as much as 385 kg N/ha (344 lb N/ac) (Shibles, 1998) and alfalfa as much as 500 kg N/ha (446 lb N/ac) (Russelle et al., 2001), compared to less than 200 kg N/ha (179 lb N/ac) for corn. Schmidt et al. (2000) demonstrated that nodulation in soybean effectively compensated with additional N when manure N was insufficient to meet crop demands; so if necessary, manure could be applied conservatively without risk of applying too little to meet crop needs.
Rate and frequency of application
As might be expected, N and P loss in runoff increases with greater frequency and rates of applied manure (Edwards and Daniel, 1993b; McDowell and McGregor, 1984). Although rainfall intensity and duration, as well as when rainfall occurs relative to applied manure, influence the concentration and overall loss of manure N and P in runoff, the relationship between potential loss and application rate is critical to establishing environmentally sound nutrient management guidelines. Also evident is that the effect of applied manure on increasing the concentration of P in surface runoff can be long lasting. For instance, Pierson et al., 2001 found that a poultry litter application tailored to meet pasture N demands elevated surface runoff P for up to 19 months after application. Although few studies have evaluated the loss of P in surface runoff as a function of application frequency, more frequent manure applications can be expected to rapidly increases soil P (Haygarth et al., 1998; Sharpley et al., 1993; 2005; Sims et al., 1998), with a concomitant increases in runoff P loss.
Intensity and duration of grazing
As beef grazing of pastures is an important component of animal production in many regions of the MARB, careful management of grazing is needed to minimize P loss and water quality impacts. The localized accumulations of P where manure is deposited can saturate the P sorption capacity of a soil, increasing the potential for P loss from grazed pastures in runoff or drainage waters. However, at a field and watershed scale, it is likely that critical stocking factors, such as density and duration, will influence both hydrologic and chemical factors controlling P transport. For example, Owens et al. (1997) found that decreasing grazing density and duration dramatically reduced runoff and erosion from a pastured watershed in Ohio. Clearly, increased runoff and erosion with grazing will enhance the potential for P loss. In Oklahoma, Olness et al. (1975) found that P losses were greater from continuously (4.6 kg P/ha/yr or 4.1 lb P/ha/yr) than rotationally grazed pastures (1.3 kg P/ha/yr or 1.2 lb P/ha/yr). In fact, P losses with continuous grazing were greater than from alfalfa or wheat (2.7 kg P/ha/yr (2.4 lb P/ha/yr); Olness et al., 1975). However, the work of Owens et al. (1997) does show that when management is changed, the impacts of the previous grazing impacts were not long lasting, changing within a year. Even so, there is a need to determine critical stocking densities and durations as a function of grazing management.
By observing four pastured dairy herds with stream access over four intervals during the spring and summer of 2003 in the Cannonsville Watershed south central, New York, James et al. (2007) were able to estimate fecal P contributions to streams. In the herds observed, on a per cow basis, cattle were especially likely to defecate in the stream, although they spent a small proportion of their time there. On average, approximately 30% of all fecal deposits expected from a herd were observed to fall on land within 40-m of a stream, and 7% fell directly into streams. Although amenities in pasture (such as water troughs, feeders, salt, and shade located away from the stream) did affect where cattle congregated, the stream demonstrated a consistent draw.
Using spatial databases of streams, pasture boundaries, and animal characteristics (i.e., number of cattle, time in pasture, and type of cattle [heifers versus milk cows]) for 90% of the dairy farms in the Cannonsville watershed, approximately 3,600 kg (7,940 lb) of manure P are estimated as deposited directly into streams with 7,650 kg (16,900 lb) deposited in pasture near streams (<10 m) from the 11,000 dairy cattle in the watershed. At this magnitude, P loadings represent a significant environmental concern, with in-stream deposits equivalent to approximately 12% of watershed-level P loadings attributed to agriculture (Scott et al., 1998). Riparian shade can also attract grazing cattle and influence P loss in stream flow.
1 The areal extent of the full hypoxic region has not been mapped with sufficient frequency to completely understand its temporal variability. The limited number of observations that have been taken more than once per year suggest that the hypoxic region reaches its maximum extent in late summer. There are physical and biological reasons to expect such a pattern of temporal variation but available data provide a conservative estimate of the maximum extent of hypoxia. The actual areal extent may be larger than estimated.
2 It is important to recognize that these studies assume a perfectly efficient water quality trading program with no trading restrictions; current water quality trading programs do not match the modeled system.
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