Final report for GNC17-242
Annual forages provide a valuable grazing resource for producers; however, annuals are prone to accumulating nitrate and toxicity can be a potential challenge. There are multiple publications regarding nitrate toxicity, but few, if any, address grazing high nitrate forages. There is variability on what amount of nitrate is considered toxic to cattle, and information is not available on the frequency producers experience toxicity when feeding annual forages. A literature review provided insight to how the guidelines on nitrate toxicity were established. The guidelines on nitrate toxicity used today are very similar to what was established in the 1940’s and 1960’s when researchers used nitrate salts to determine the toxic dose. Multiple factors impact the risk of nitrate toxicity in individual scenarios and indicate that the risk of nitrate toxicity when grazing annual forages is less than the risk when feeding hayed annual forages. To better understand the risk when grazing forages, cooperating beef producers were contacted and blood and forage samples were taken to observe how affected cattle were while grazing various concentrations of nitrate. To understand the incidence of nitrate toxicity in the North Central Region of the U.S., a survey was distributed through the “UNL BeefWatch” newsletter to producers. Though producers appeared concerned about nitrates in annual forages, only 38% have experienced an issue. Management decisions to test annual forages for nitrates did not change if a producer had previously experienced toxicity. Producers tended to experience nitrate toxicity more often when grazing (31%) compared to feeding hay (21%). This data agreed with a dataset of samples submitted to Ward Laboratories, in which 48% of fresh brassica samples, 23% of fresh annual grasses, and 5% of dry annual grasses analyzed would have been considered at risk for causing toxicity. However, the increased incidence of toxicity in pasture is smaller than expected based on the large proportion of fresh forages sampled and submitted to the commercial laboratory and considered toxic. Some mitigation factors may explain differences in toxicity risk for animals grazing compared to animals fed annual forage hay. Understanding these factors and the cost of not utilizing the forage is important for management decisions. Although these forages pose a risk of toxicity, they provide a high quality feed source.
Grazing cover crops can be an avenue to obtain economic value while achieving environmental stewardship. This project addresses one of the potential barriers to grazing cover crops as high nitrates levels have been observed in the forage species used. However, the perceived risk when grazing cover crops/annual forages may be greater than actual risk. The reduced rate of intake and increased selectivity when grazing, the decreased rate of nitrate release into the rumen with fresh forages, and the high-energy content of the diet may decrease the potential for toxicity.
The objective of this project was to: Increase the understanding of nitrate toxicity potential in grazed cover crops/annual forages.
This project evolved to include multiple approaches to collect data. These included a literature review, producer cooperation to collect blood and forage samples in the spring and fall, a survey distributed to producers, and a collaboration with a commercial laboratory.
Measuring methemoglobin in the blood with various nitrate concentrations in grazed pastures
The first approach was to contact producers planning to graze annual forages in the spring or fall prior to turn out. Four to seven days after cattle began grazing the annual forage, a subsample of the cattle (~10 head) were used to collect blood samples, which were later analyzed for methemoglobin content (Evelyn and Malloy, 1938; Leahy and Smith, 1960, Landeria et al,, 2002). The timing of blood collection was intended to achieve collection near peak methemoglobin concentration (Kemp et al., 1977). The day cattle were bled, three different quality samples of the annual forages were collected. One sample per forage species by clipping at ground level throughout the field in a zig zag pattern, the second sample was clipped at ground level in approximately a 4 foot by 4 foot area, and a third sample was taken by plucking forage by hand in a zig zag pattern throughout the field in an attempt to mimic what cattle were consuming. Samples were froze for a minimum of 24 hours before being dried, ground, and analyzed for nitrate content. Nitrate-nitrogen was determined on a dry matter basis (ppm NO3-N DM) using a nitrate ion selective electrode. One gram of dried, ground sample was continuously mixed in 40 ml of pH 7 distilled water at room temperature with a rocker for 30 minutes before measuring. A standard curve was developed using known nitrate standards to calibrate the electrode prior to sample analysis (Anderson and Case, 1999).The methemoglobin content and nitrate content were compared to see if a relationship can be identified, and used to determine toxicity potential. All protocols involving animals were approved by the University of Nebraska- Lincoln Institutional Animal Care and Use Committee.
Retrospective analysis to see if previously grazed forages contained elevated nitrate concentrations
When analyzing the annual forages collected on cooperator farms for nitrate concentration, stored forage samples that had been dried and ground from other grazing trials were also measured for nitrate content. This was done to observe if animals had grazed elevated nitrate concentrations in the past without experiencing any health consequences.
Survey of beef producers addressing nitrate toxicity in annual forages
A survey was distributed online, and a link was emailed to subscribers (n = 1182) of the “UNL beefwatch” newsletter, and participants of extension meetings in Kansas and Nebraska were encouraged to fill out the survey. The survey consisted of 16 multiple choice questions, asking about production decisions made regarding testing and use of annual forage pasture or hay and demographics.
Commercial Laboratory Collaboration Evaluating Average Nitrate Accumulations
Annual forage samples (n = 1957) submitted by producers to Ward Laboratories, Inc. (Kearney, NE) for nitrate analysis during 2016 and 2017 were summarized. Samples were initially sorted into “fresh” (< 26% DM) and “dry” (>84% DM) categories. The fresh samples (n = 443; 18.2% DM; SD ± 4.6%) were classified into five species groups based on their label: 1) brassica (turnip, radish, collard; n = 63); 2) mixture (cover crop mix or multiple annual forage species; n = 34); 3) small grain forages (oat, rye, triticale, wheat, barley; n = 70); 4) millet (pearl, foxtail or German; n = 40); or 5) sorghum/sudan (cane, milo or sorghum, sorghum-sudangrass, sudangrass; n = 236). The dry samples (n = 1514; 87.0% DM; SD ± 2.2%) were sorted into four species groups based on their label: 1) oats/pea mix (oats/pea mix; n=60); 2) small grain forages (oats, rye, barely, triticale, wheat; n = 595); 3) sorghum x sudangrass (cane, sorghum, sudangrass, milo, sorghum x sudangrass; n = 532); and 4) millet (pearl, foxtail, German; n = 327). Samples were evaluated for differences in average nitrate-nitrogen (NO3-N) concentration within moisture type. Additionally, for the species with samples in both fresh and dry groups (sorghum x sudangrass, millet and small grain forages) nitrate concentration was compared to evaluate differences among moisture types.
Within moisture type, the proportion of the samples in each species category that fell into nitrate toxicity recommendation ranges was also evaluated. These nitrate toxicity ranges were: 1) Safe (<1400 ppm NO3-N DM); 2) Marginal (1400-2100 ppm NO3-N DM); 3) Caution (2100-5000 ppm NO3-N DM); and 4) Toxic (>5000 ppm NO3-N DM). Bradley et al. (1940) recommended that 2100 mg NO3-N/kg DM be set as the threshold above which toxicity may occur. Many extension publications from the Midwestern U.S. use values close to 2100 ppm NO3-N DM as the threshold where the forage is potentially toxic and would need to be managed accordingly. Table 1 provides examples of multiple extension programs and their current nitrate toxicity recommendations.
Individual Field evaluation
Additionally, six fields planted to a small grain forage-brassica mixture in late summer were sampled as fresh forage in late fall and evaluated to determine if species accumulation of nitrates differed when grown under identical conditions. These mixtures included oats or cereal rye planted with turnips and/or radishes. Samples were obtained by randomly selecting individual species throughout the field, clipping small grain forages to ground level, and pulling the whole brassica plant up and separating the top (leaves + stem) from the root. All samples were dried in a 60 °C forced air oven and ground to a 1-mm screen in a Wiley mill (Thomas Scientific, Swedesboro, NJ). Nitrate-nitrogen was determined on a dry matter basis (ppm NO3-N DM) using a nitrate ion selective electrode. One gram of dried, ground sample was continuously mixed in 40 ml of pH 7 distilled water at room temperature with a rocker for 30 minutes before measuring. A standard curve was developed using known nitrate standards to calibrate the electrode prior to sample analysis (Anderson and Case, 1999).
Given the limited number of blood collections, no statistical analysis was conducted on the data comparing methemoglobin content in the blood to the nitrate content of the grazed diet.
Chi-Square analysis in SAS 9.3 (SAS Inst. Inc., Cary, NC) was used to compare the differences in survey responses regarding testing, use, and toxicity of annual forage pasture vs. hay. Binomial analysis in SAS 9.3 was used to determine 1) if those that have experienced issues with nitrate toxicity in hay or pasture differed from those that have not experienced issues in their frequency of testing those forages, and 2) if those that reported experienced issues with nitrate toxicity in hayed or grazed annual forages differ in their use of hay or pasture that tested high in nitrates.
Commercial laboratory data were analyzed using the GLIMMIX procedure of SAS 9.3. To determine effect of species on nitrate concentration within moisture type the model included species as a fixed effect. To evaluate the differences of fresh or dry forages the model included moisture type, species, and their interaction as fixed effects. To determine what proportion of the samples in each species would fall into the nitrate toxicity categories, a multinomial distribution with a cumulative logit link function was used to conduct pairwise comparisons of species within moisture type using the odds ratio function. The model for the grass and brassica sampled from the six fields, included species as a fixed effect and location as a random effect. For all analysis, effects were considered significant at P ≤ 0.05 and a tendency when P is greater than 0.05 and less than or equal to 0.10.
Unfortunately, the collections that involved bleeding cattle grazing annual forages did not have the impact we anticipated, although the information is still useful. The limited number of collections resulted in no statistics being performed. Collections included gestating cows, bred heifers, and steers. The maximum estimated diet nitrate content was 3,061 ppm NO3-N DM. The next highest diet estimate was below the 2,100 ppm NO3-N DM used as a recommended threshold in the past. The nitrate content of the diet might not have been high enough to cause the animals to experience any symptoms of nitrate toxicity. The farm with the highest methemoglobin content in the blood averaged 17.8% of hemoglobin. Although this is higher than the normal range for cattle, signs are not typically noted until 40 to 60% is reached. More collections are needed to quantify the risk of nitrate toxicity while grazing through this method in order to predict the risk.
Retrospective analysis of cover crops grazed in previous growing cattle trials
When late summer planted oats and oat-brassica mixes from previous grazing trials were analyzed for nitrate concentration, most were well above the currently cited toxicity threshold. Table 2 provides the results from measuring the nitrate concentrations in the forage. In all but one trial, the concentration exceeded 2,100 ppm NO3-N DM, indicating that they would have been considered at risk for causing toxicity. In all of these cases, no signs of nitrate toxicity were observed. It is important to note that these samples were collected by clipping the forage to ground level, and that the diet was likely initially lower than the results indicate because of this collection method. These cattle were allowed to be selective as they were set stocked, such they had access to between 45 and 60 days’ worth of forage at the start of grazing. This allows them to graze the leaf and upper parts of the plant first which are lower in nitrate than the stem, with the lower part of the stem having the most nitrate. Thus, grazing at stocking rates that allow for selectivity and self-adaptation (grazing lower nitrate plant parts and working down the plant) likely reduces toxicity potential. Some other potential mitigation factors include the high digestibility of the forage (more energy for bacteria in the rumen to use the nitrite), the fact that fresh forages release nitrate in the rumen at a slower rate than dry forages, and that there is likely a slower rate of intake when grazing compared to consuming hay. While the above information may suggest there is a lower risk of nitrate toxicity when grazing highly digestible forages at low stocking rates, it does not allow us to conclude what nitrate content would be acceptable.
Survey of cattle producers
Survey respondent demographics are illustrated in Figure 1, 2, and 3. Most respondents were from the Midwest and managed both cows and stocker/backgrounding calves. Most (71%) of producers indicated the issue of nitrate toxicity in annual forages was “very important” or “important” to them. The survey respondents were more likely (P = 0.02) to test annual forage put up as hay than annual forage that was grazed for nitrate concentrations (Table 3). However, when asked about the frequency that their annual forages tested high in nitrates the majority (90%) responded that they “never”, “rarely”, or “occasionally” test high. Despite the perceived importance, the majority of annuals used for forage by the respondents appear not to be tested and those that are tested often do not contain elevated nitrate concentrations.
Most (62%) of the producers that responded to the survey had not experienced nitrate toxicity when grazing or feeding hayed annual forages (Figure 4). There was a tendency for producers to report that they have had issues more (P = 0.09) with pasture than with hay (Table 3). More producers were likely (P < 0.01) to use hay that tests high for nitrates than pasture (Table 3). This may be due to the ability to mix high nitrate hays with other hays or feeds to dilute nitrates in the diet as is often recommended in extension publications. Many extension documents do not provide advice on how to use high nitrate pastures. Despite the relatively low likelihood of testing and low incidence of toxicity, it does appear that producers are concerned about the potential for toxicity and use test results to make decisions.
For both fresh and hayed forages, data reported in Table 4 indicate that if a producer experiences nitrate toxicity, the experience does not influence them to implement regular testing of annual forages for nitrate content as a prevention strategy. When comparing producers that have experienced nitrate toxicity in the past when grazing or feeding hay to those that have not, there was not a significant difference (P ≥ 0.28) in the likelihood that they would graze or feed hay that tested high in nitrates in the future (Table 4).
Annual forage nitrate content
Although producers in the Midwest region are concerned about nitrate toxicity, only a small percentage regularly test forages for nitrates. Since most appear to not test forages, the forages submitted to Ward Labs may only be representative of nitrate levels when the producer is concerned about increased concentrations. Within the fresh forage samples brassicas contained the most (P < 0.01) nitrate (Table 5). The cover crop mixtures, sorghum x sudangrass, millet, and small grain forage did not differ (P > 0.05) in nitrate concentration. The frequency that each of the species was classified into the nitrate toxicity ranges based on extension recommendations are illustrated in Figure 5. Brassicas exceeded 2100 ppm NO3-N DM in 47.6% of samples and the odds ratio indicated that they were 3 to 5 times more likely to be above this threshold than the other species categories (Table 6). The remaining species ranged from 20-28% of the samples being above 2100 ppm NO3-N DM, indicating that there is still a reasonably high likelihood that these fresh annual forage samples could be considered toxic using current guidelines.
In the dry samples from the commercial lab data set, sorghum x sudangrass and oats/pea did not differ (P = 0.78) for nitrate concentration (Table 5). Millet contained less (P < 0.01) nitrate than sorghum x sudangrass but did not differ (P = 0.19) from oats/pea. Small grain forages contained the least nitrate (P < 0.05) compared with other species. Figure 6 depicts the frequency that the dry samples fit into nitrate toxicity ranges based on extension recommendations. Small grain forages only exceeded 2100 ppm NO3-N DM in 2.5% of samples and the odds ratio indicated that they were 2.0 to 3.6 times less likely than the other species to contain nitrate concentrations above 2100 ppm NO3-N DM (Table 7). Both millet and oats/pea mixtures exceeded the 2100 ppm NO3-N DM in 8% of samples and did not differ in the likelihood that they would contain nitrate concentrations above this threshold. Sorghum x sudangrass exceeded the exceed 2100 ppm NO3-N DM in 11.5% of samples and was 1.6 fold more likely to exceed this threshold than millet (Table 7).
There were nearly three times as many dry samples (>84% DM) submitted for analysis compared to fresh (< 26% DM) samples (n = 1514 vs. n = 443, respectively). This indicates producers submit more hayed forages for analysis than fresh forages, an observation that was also made in the survey of producers. Fresh samples (1,321 ppm NO3-N DM) had greater (P < 0.01) nitrate content than dry samples (637 ppm NO3-N DM).
The six field collections with fresh small grain forages and brassica mixes agreed with the dataset from the commercial laboratory. Small grain forages (161 ppm NO3-N DM) contained less (P < 0.01) nitrate than brassicas tops and roots. Within brassica species, there was no difference (P ≥ 0.77) between the top and roots. However, radish top (9,248 ppm NO3-N DM) tended (P = 0.06) to have greater nitrate than turnip tops (5,932 ppm NO3-N DM) whereas radish roots (9,073 ppm NO3-N DM) were numerically, but not statistically different (P = 0.12) from turnip roots (6,354 ppm NO3-N DM) in nitrate content.
Genetic and management differences likely account for the difference in species accumulation. A review by Garnett et al. (2009) discusses differences in root systems and the influence on nitrate accumulation. Root size, length, surface area, and present transporters affect nitrate uptake. Available N and internal regulations influence the activity of N transporters as well (Garnett et al., 2009). Brassicas are often included in nitrate accumulator lists due to their tendency to accumulate nitrate (Maynard et al., 1976; Provin and Pitt, 2003). Both the commercial laboratory data set and comparison of samples taken from brassicas and grasses grown together in the same fields suggest that there is an increased risk for brassicas to contain concentrations of nitrate than would be deemed toxic. However, the high sugar and high digestibility of brassicas may help mitigate some of the risk of nitrate toxicity. Research on the risk to nitrate toxicity when grazing high nitrate brassicas would be valuable.
Toxicity potential with grazing vs. feeding hay
Producers’ responses suggest that they experience toxicity more often in grazed forages than hayed which would support the data that fresh samples accumulate higher concentrations of nitrate. However, it is important to note that the incidence of toxicity in the survey data was not extremely elevated despite the fresh grass samples in the commercial laboratory data set containing twice as much nitrate as the dry grass samples. When only the species in both the fresh and dry groups (small grain forages, millet, and sorghum x sudangrass) are considered, 23% fresh annual grasses and 5% of the dry annual grasses were above 2,100 ppm NO3-N DM. When weighted based on the number of samples received by the commercial lab, 27% of fresh and 7% of dry samples were above 2,100 ppm NO3-N DM. The increased nitrate concentrations in fresh forages may be explained by the fact that maturity impacts nitrate concentrations. Immature, vegetative forage contains more nitrate than mature forage (Crawford et al., 1961) and fresh forages are often grazed in vegetative stages while hayed forages are harvested at a more mature stage in order to increase yield per harvest.
The nitrate toxicity guidelines were first developed by Bradley et al. (1940) by orally dosing cattle with nitrate salts. Later, Crawford et al. (1966) showed that nitrate in hay and top dressed on hay is three times less toxic than when given as a drench. This may be partially explained by the slower rate of nitrate availability in the rumen when cattle consume nitrate with the forage rather than receiving it all at once in a drench. Later, Kemp (1982) reported that fresh forages have a lower risk of nitrate toxicity, with the amount of nitrate in fresh forages having to be almost double, at the same amount of DM intake per meal, to achieve the same level of toxicity (methemoglobin) as hay. In an in vitro study, Geurink et al., (1979) found a substantial difference in the rate nitrate was available when provided in a fresh plant cell compared to a dry plant cell. Within 20 minutes, 80% of the total nitrate was available when grass hay was submerged in water. At the same time, only 30% of the nitrate was available when fresh chopped grass or turnips were submerged. This may explain the apparent difference in toxicity potential between fresh and dry forages. However, current nitrate toxicity guidelines do not differentiate between fresh and dried forages. Furthermore, when grazing, cattle selectively choose to consume the leaf portion of a plant before consuming the stem (Chacon et al., 1978) which contains the greatest concentration of nitrate (Crawford et al., 1966). Thus, grazing at lower stocking rates to allow for selectivity can reduce toxicity potential (Bolan and Kemp, 2003). Additionally, increased diet digestibility and energy supplementation have both been shown to reduce the risk of toxicity (Burrows et al., 1987; Sapiro et al., 1949). Annual forages, particularly brassicas and late summer planted small grain forages are highly digestible (Coblentz and Walgenbach, 2014; Contreras-Govea and Albrecht, 2006; Villalobos and Brummer, 2015). Given the relatively high proportion of fresh annual forages tested that would be considered potentially toxic using traditional guidelines, and the fact that grazing situations, especially when grazing highly digestible forages, may have lower toxicity potential, more research to develop guidelines and management strategies for grazing annual forages would be beneficial.
Educational & Outreach Activities
- Factsheets/educational tools
- Nitrate Concentrations of Annual Forages Grown for Grazing in Nebraska. 2019. Nebraska Beef Cattle Report. https://beef.unl.edu/documents/2019-beef-report/MP106_pg045_Lenz_et_al.pdf.
- Reducing Nitrate Concerns When Grazing Forage Cover Crops. 2018. UNL CropWatch electronic Newsletter. https://cropwatch.unl.edu/2018/reducing-nitrate-concerns-when-grazing-forage-cover-crops
- Press articles
- The oddity of forage nitrates. Hay and Forage Grower. July 2017. https://hayandforage.com/article-2048-the-oddity-of-forage-nitrates.html
- It’s time to rethink nitrates. Hay and Forage Grower. February, 2019. https://hayandforage.com/article-2332-It%E2%80%99s-time-to-rethink-nitrates.html
- Nebraska Extension Beef Educator In-service Training
- May 2017 (20 educators/specialists in attendance)
- May 2018 (30 educators/specialists in attendance)
- Presentations at Nebraska Extension producer education workshops
- Mead, NE. April, 2018 (15 beef producers in attendance)
- Clay Center, NE. January, 2019 (40 beef producers in attendance)
- Wilber, NE. February, 2019 (14 beef producers in attendance)
- Oral Presentation at Scientific Meeting
- Nitrate Concentrations of Annual Forages Grown for Grazing in Nebraska. American Society of Animal Science and Canadian Society of Animal Science annual meeting. Vancouver, B.C. July, 2018. https://academic.oup.com/jas/article/96/suppl_3/196/5234146 (10 people)
- Poster Presentations
- Nitrate Toxicity Risk in Grazed Annual Forages. High Plains Nutrition and Management Roundtables. Laramie, WY. October, 2108 (35 industry professionals)
- Reevaluating Nitrate Toxicity Potential when Grazing Annual Forages. American Forage and Grassland Council. St. Louis, MO. January, 2019. https://www.afgc.org/i4a/doclibrary/index.cfm?category_id=20 . (20 producers/industry professionals)
- Management and Risk of Nitrate Toxicity in Annual Forages: Results of a Beef Cattle Producer Survey. Midwest Meeting of the American Society of Animal Science. Omaha, NE. March, 2019. (10 industry professionals)
This project expanded our knowledge on utilizing annual forages as a feed source, especially as it pertains to managing the associated risk of nitrate toxicity when grazing these forages. As annual forages are utilized as cover crops, their purpose and ability to scavenge nutrients and improve critical soil properties are essential in our industry going forward, especially if our industry will thrive for generations to come. Utilizing livestock to graze these annual forages allows producers to offset the cost of establishing the forage, and continue this sustainable practice. Aside from the economic benefit of grazing these cover crops to cover seed and planting costs, grazing gives livestock producers an additional feed resource when faced with a continual decline of pasture availability.
This project will impact the use of annual forages in the future as it clearly addresses the risk of nitrate toxicity when grazing annual forages, and how to evaluate the level of risk in individual scenarios. Nitrate toxicity has occurred in annual forages in the past, and the risk is there for both grazed and hayed forages. However, the guidelines used to advise producers on the risk of toxicity are outdated, and are not black in white in their applicability to each unique operation. This is especially true when a fresh, high quality forage is grazed, rather than an animal being limit fed a low quality, high nitrate harvested hay.
More than anything, a better appreciation for the complexity of nitrate toxicity in cattle was gained. A critical outcome of this project was a better understanding of how many producers are concerned about, test forages for, and have experienced nitrate toxicity when utilizing annual forages as either a hayed feed, or as a grazing resource. By going through literature, it became apparent that the current recommendations for nitrate toxicity are based on early studies that found the lethal dose of nitrate by dosing nitrate salts through a drench or bolus, or by topdressing hay with a nitrate salt solution. Both of these methods cause the nitrate to be immediately available for rumen microbes, and are not directly comparable to a slowly consumed nitrate containing diet. A limited number of tested animals also cause variable results, and within those studies, there is data supporting that a fresh, high quality, grazed diet has a lower risk of toxicity than what recommendations indicate. In addition, understanding the species that accumulate nitrates most often is important going forward and advising producers. Though progress was made, more work can be done on nitrate toxicity even though this topic was studied as early as 1939 (Bradley et al.). More research is needed to refine the recommendations, and help producers confidently make decisions on forage utilization on their operation.
Producers that attended presentations were more confident in making decisions regarding grazing cover crop/annual forages with varying levels of nitrate with 91% of producers responding that they had moderate or significant improvements in knowledge and 75% plan to modify their management to reduce the potential of nitrate toxicity.
Cattle grazing fresh annual forages tolerate a higher nitrate load than animals being fed a dried, harvested annual forage. A few basic management practices are recommended to further reduce risk when grazing higher nitrate forages.
When grazing higher nitrate forages…
- Slow them down. Make sure cattle are full before putting them on fields. Regardless of the nitrate level, a good management practice is to fill cattle up with hay before turn-out. Keep them full. If intake becomes restricted at any point (forage runs out or weather impedes grazing) fill them up on lower nitrate hay, before they go back to grazing the high nitrate forage.
- Gradual adaptation is a key management strategy to minimize risk when using high nitrate forages. The bacteria in the rumen capable of degrading nitrate to ammonia for bacterial protein synthesis will increase in numbers when nitrate is available to them. Adapted animals can safely be fed higher levels. To adapt the cattle, start by grazing the lowest-nitrate fields and then work up to the highest.
- Graze higher nitrate fields lightly to allow animals to selectively graze plant parts that are lower in nitrate concentration.
- Consider grain supplementation while feeding cattle high nitrate lower quality forages such as mature sorghum x sudangrass hybrids or pearl millet. This will supply energy for rumen microbes to convert nitrate into bacterial protein and minimizes nitrite (the intermediate) production. Brassicas, such as turnips and radishes are highly digestible and as such may provide enough energy to allow for increased microbial protein synthesis. Grain feeding may be of limited benefit when grazing high quality cover crops.