«University of California Division of Agriculture and Natural Resources Committee of Experts on Dairy Manure Management September 2003 February 2004, ...»
Unless solids buildup in a pond is managed to be negligible using agitation or some other method, a significant fraction of the non-soluble phosphorus may also build up in the pond sludge. At some point, this sludge will be land applied. Finally, the above examples only consider liquid manure, but do not take into account contributions from dry manure and separated solids. These materials can be expected to contain significant amounts of phosphorus but smaller amounts of potassium.
In a different study, Campbell-Mathews et al. (2001a) surveyed the nutrient contents of dairy wastewater lagoons in the Central Valley. Assuming crop harvest removal of 105 lbs P2O5 per acre (118 kg ha-1) and 250 lbs K2O per acre (281 kg ha-1) in 30 tons fresh weight per acre (67 mT ha-1) of harvested silage corn, and assuming that these liquid manures had been used to meet N uptake requirements, 21% of the reported cases would have resulted in applications exceeding the expected crop uptake of P2O5, and 75% of the cases may have exceed the expected crop uptake of K2O. This hypothetical exercise assumed that the crop harvest N removal is met with the total ammonium and 50% of the organic nitrogen in each of approximately 130 liquid manure samples.
In monitoring data from a shallow groundwater monitoring well network (Harter et al., 2002), dissolved total phosphorus in monitoring wells downgradient from fields averaged just above 1 mg L-1 and much less than 1 mg L-1 in monitoring wells downgradient from storage lagoons and corrals. The higher concentrations underneath fields is likely due to the low sorption capacity of the sandy soils at the particular sites and the lack of organic material at the soil surface (relative to the storage lagoon and corral surfaces). Phosphorus tends to strongly sorb to clay particles and organic matter. Hence, even in a sandy aquifer, P is unlikely to be transported large distances off-site. However, under tile-drainage conditions or in the presence of streams in the immediate vicinity of the dairy, phosphorus concentrations at the observed levels may significantly impact surface water quality if groundwater discharges into surface water bodies. It is unknown, whether similar levels of dissolved total P would be observed in regions where soils have higher silt and clay content or in regions with deeper water table.
Soil and tissue sampling could be used effectively to monitor excesses and deficiencies of both P and K. Currently, many dairymen are observing excessively high K concentrations in forages (3.0%) where lagoon water or manures have been applied. This may lead to depressed dietary magnesium levels, affect animal health and in severe cases cause death of cattle, particularly lactating or pregnant cows (Grunes, 1973).
6.3 Summary Currently available literature data and the limited amount of recent research in the San Joaquin Valley on the nutrient cycle of P and K in dairies does not allow for a conclusive finding on the question of phosphorus and potassium status in field soils used for application of animal waste.
However, trends are certainly apparent that dairyman will need to monitor the increased application of both P and K to determine the impact on forage nutrient levels and animal performance.
7.1 Introduction In 1973, the UC Committee of Consultants Water Quality Task Force estimated that dairy cows in the Santa Ana Basin, Central Valley and North Coast excreted 1.85, 1.56, and 1.97 lbs (840, 708, and 894 g) of dissolved mineral salts cow-1 day-1, excluding the N containing mineral constituents. In 1973, an average dairy cow was fed 42 to 44 lbs (19 to 20 kg) of dry matter (DM) daily. In 2003, a typical cow was fed 44 to 51 lbs (20 to 23 kg) of DM daily, with some lactating groups with high milk production being as high as 66 lbs day-1 (30 kg day-1). While the composition of the rations has changed over this period, the basic ingredients that constitute the rations have remained similar. The inorganic mineral contents of the feed ingredients such as alfalfa hay, grains and other others also remained relatively constant. Salt excretion levels are therefore expected to have increased proportional to the DM consumption.
In this chapter, we examine currently known excretion levels based on dietary considerations (section 7.2), and various estimates of field salt loading (section 7.3). Section 7.4 considers the environmental effects of salt loading and section 7.5 the effect of dairies on the regional salt balance in the Central Valley. These last two sections address the question of what constitutes a “reasonable” salt loading rate.
7.2 Salt Intake and Excretion by Lactating Dairy Cows in California 7.2.1 Salt Excretion by Lactating Dairy Cows in California Salts and minerals in dairy manure derive primarily from minerals in the feeds that constitute the ration, and mineral constituents added to the ration to meet the mineral requirements of the target animal group. While a dairy cow may drink up to 50 gallons (190 l) of water day-1, the dissolved mineral load from the drinking water itself is relatively minor (less than 5%) and not expected to contribute significantly to the overall load, except in exceptional cases. For example, Castillo et al. (2005) reported, based on a survey of lactating dairy cows on 51 randomly selected dairy farms in the California Central Valley, that an average of 16% and 8% of the Na intake and Cl intake, respectively, came from drinking water.
There have been no organized survey studies of dairy rations in California, save the groups of lactating cows described in Chapter 2. Even here, the ability to estimate salt intake is limited to the groups in the Petaluma, Tulare and Modesto areas due to a lack of definition of the added mineral premix in the Chino herd. Currently, salt intake and excretion data are only available for specific ions, namely sodium (Na+), potassium (K+) and chloride (Cl-). Excretion rates for other salts are currently not available although they can be estimated for specific rations (chapter 2).
Based upon the average of the three herds (six groups of cows) the average level of Na+, K+, and Cl- in the rations was 2.99% of DM. Assuming an average DM intake of 51 lbs (23 kg) cow-1 day-1, average Na+ + K+ + Cl- intake would be approximately 1.52 lbs (688 g) cow-1 day-1.
According to mass balance calculations on these groups of lactating dairy cows described in Chapter 2, the UC Dairy Animal Waste Estimator calculates that 82, 91, and 76% of Na+, K+, and Cl-, respectively, will be excreted in feces and urine or, on average, about 85% of the total Na+ + K+ + Cl- intake. Thus of the 1.52 lb (688 g) d-1 consumed, about 1.29 lbs day-1 (585 g dayor 392 lbs (178 kg) cow-1 per 305 days lactation] of Na+ + K+ + Cl- will be excreted. That these values are less than values reported by the 1973 UC Committee of Consultants Water Quality Task Force partially reflects the fact that the 1.29 lbs day-1 is Na+ + K+ + Cl- only and not total salts. Based on dietary considerations, total salt excretions are typically more than 50% higher and as much as 100% higher than the Na+ + K+ + Cl- excretions alone (see, for example, the survey reported by Castillo et al., 2005).
7.2.2 Salt Intake and Excretion by Dry Dairy Cows in California
There are no equivalent California data for dry cows comparable to those presented for lactating cows in Chapter 2 and above. However, in general, dietary salt levels are lower (by approximately 20%) in rations of dry dairy cows and feed intake levels are substantially less.
However there is no milk being exported, hence there is no salt export in milk. Thus, for average California dry cows consuming 26 lbs (12 kg) of DM cow-1 day-1., a reasonable average Na+ + K+ + Cl- intake would be 0.68 lbs (287 g) day-1 of which 95% would appear in manure, or 0.60 lbs (273 g) day-1 [35 lbs (16 kg) cow-1 day-1 dry period].
7.2.3 Factors that Affect Salt Intake by Dairy Cows in California
Salt intake by dairy cows can be expected to vary to a great extent among cows, and among groups of cows on different dairies, due to differences in DM intake as well as dietary salt levels.
Differences in dietary salt levels can be caused by differences in the salt levels (especially K) of the forages in the ration as well as formulated minimum levels of salts among consulting nutritionists. In addition, it is still not uncommon to find ‘free-choice’ salt (NaCl only or trace mineral enhanced NaCl) provided either loose or in pressed blocks. This practice can dramatically increase salt intake and excretion.
7.3 Field Salt Loading Salt load onto the field is defined as mass of total mineral salts (total dissolved solids, TDS) added to a unit of land area over a given time period. In dairy wastewater, the TDS is composed predominantly of the major inorganic cations (Ca2+, Mg2+, Na+, K+, NH4+) and anions (Cl-, SO42-, HCO3-, and NO3-) but also includes a significant amount of organic anions, most of which will ultimately mineralize to the bicarbonate anion (HCO3-). In irrigation water, more than 95% of the TDS is due to Ca2+, Mg2+, Na+, K+, Cl-, SO42-, HCO3-.
Salt loads are unavoidable on irrigated land as the irrigation water invariably contains dissolved minerals which will be concentrated when the applied water evaporates and transpires. Salt inputs (TDS) from the irrigation water for year-round production of forage crops in the Central Valley is on the order of 2858 lbs ac-1 (3,200 kg ha-1). This estimate is based on the assumption that approximately 1 m (3.3 ac-ft ac-1) of irrigation water is applied per year and that the electrical conductivity (EC) of irrigation water is on the order of 0.5 mmho cm-1 (dS m-1). This EC corresponds to a total dissolved solids concentration of approximately 320 ppm (mg L-l). In most irrigation water in the eastern San Joaquin Valley and in the Sacramento Valley, the majority of dissolved solids are Ca2+, Mg2+, HCO3-, and SO42-. Irrigation water, particularly surface water, is relatively low in Na+, Cl-.
On dairies, field salt loading and subsequent salt loading to groundwater (the amount of total dissolved solids annually recharged to the water table per unit land area) can be estimated by various methods. We estimated approximate field salt loading on manure-treated land a) based on average excretion rates of Na+ + K+ + Cl- and nitrogen and nitrogen application rates in fields (holistic, farm-based mass-balance approach) and b) based on field reconnaissance data of liquid manure water quality (lagoon water quality approach). Using chemical speciation models, we derived groundwater loading rates from the land application and crop nutrient uptake data and compared those to measured groundwater quality data.
7.3.1 Mass Balance Estimation
The use of dairy manure in nutrient management imposes a heavier salt load on croplands receiving the manure than on croplands receiving commercial fertilizer only. Under the yearround forage production scenario for the Central Valley, the crop nitrogen uptake may be much as 268 – 625 lbs N ac-1 yr-1 (300 - 700 kg N ha-1 yr-1). Table 7-1 gives the amount of Na+, K+, and Cl- salts excreted and applied if the nitrogen applied is from dairy manure only. The values in Table 7-1 assume that manure application rates match the recommended average NIR of 1.4 to 1.65 (see chapter 5) and that the total N losses in flush lanes, corrals, and the lagoon are in the range of 22% to 50% (somewhat broader than the range given in chapter 4). Low values assume lowest estimated N losses and lowest average NIR, while high values assume highest estimated N losses and highest average NIR. Salt excretion is based on the estimates given above.
Table 7-1: Approximate range of nitrogen and Na+ + K+ + Cl- excretion [kg/ha/year] as a function of the annual crop nitrogen uptake [kg/ha/year]. Ranges of NIR and N losses assumed are described in chapters 4 and 5.
Unless manure solids are exported from a dairy facility, the long-term salt application rates within the dairy are equal to the salt excretion rate. Other than through solids export, there are no net losses of Na+, K+, and Cl- (or other salts) in the dairy manure pathway. On facilities with significant solids separation, solids may be temporarily recycled for bedding, but will ultimately be applied to fields (see section 3.3.3 of this report).
Given the values in Table 7-1, the salt load in high production forage systems receiving dairy manure application is expected to be approximately 50% to over 100% higher than on land in regular crop production (with irrigation water as the sole sources of salinity). Note that Table 7does not include all salts. Ca2+, Mg2+, SO42-, HCO3- and nitrogen salts in manure also contribute significantly to the salt loading. On the other hand, crops have a significant capacity to take up potassium and nitrogen salts (in form of NH4+ or NO3-) (see chapter 5).
7.3.2 Groundwater Loading Estimation using Chemical Speciation Model
The above estimates represent salt loading to the land surface. The actual salt inputs to groundwater will be dependent on the compositions of the inorganic chemical constituents in the manure, the capacity of the crop to take up salts (predominantly N and K), and chemical precipitation-dissolution reactions in the soil. Under appropriate conditions, certain minerals may precipitate out of the solution and thus reduce the inputs. It is also possible that the oxidation of ammonia to nitrate will induce dissolution of precipitated minerals, thus increasing the amount of salts being leached.