Late season applications of nitrogen are made by many Colorado farms that produce fresh market russet potatoes. This graduate student research project was intended to determine if the applications provide a benefit to growers or if returns are higher under a more sustainable fertilizer program in which no late nitrogen is applied. Substantial negative effects on tuber quality were not observed from the applications. However, economic analysis based on yield indicated that Canela and Norkotah CO8 could produce lower returns due to excessive late nitrogen, while Rio Grande had slightly higher returns one year with late nitrogen and no difference the second year.
Economic returns from fresh market potato crops are affected not only by conditions and diseases during the growing season but also by factors occurring during harvest and storage. Potato growers and shippers who sell to the fresh market often have significant economic losses from harvest damage as well as from pressure bruising and moisture loss (shrinkage) in storage. Immature potato tubers are more prone to shrinkage of potatoes in storage and more susceptible to bruising (Thornton, 1998). A primary cause of immaturity of potatoes at harvest is excessive nitrogen fertilization during the late growing season. Increased nitrogen applied during the later part of the growing season can increase potato yields for some cultivars but may delay harvesting, risking frost damage in areas with a short growing season. Tuber damage may increase during harvest because of tuber immaturity caused by over fertilization. Potato farmers in southern Colorado also use late growing season applications of nitrogen to reduce early blight (Alternaria solani) damage which will destroy the vines of susceptible potato varieties prior to development of sufficient yield. However, with increases in the cost of nitrogen fertilizers and a growing awareness of sustainable growing methods, many western U.S. potato growers are interested in reducing the nitrogen they apply. Research into reducing agricultural inputs such as nitrogen fertilizer is important to improving the sustainability of agricultural systems (Spiertz 2009).
While nitrogen fertilization research has been conducted many times for potatoes, results indicate that the optimum timing and rates are variable depending on cultivar (Love et al.,2005 ). Tuber moisture loss during storage has been observed as a consequence of excessive late season nitrogen and from harvesting of immature tubers or tubers with poor skin set (Thornton 1998 and Olsen 2003). Tuber moisture loss causes further economic losses due to increased susceptibility to pressure bruising of potatoes stored for the fresh market (Jayanty, 2009). This two-year Western SARE funded graduate research project focused on understanding the effects of different rates of late season nitrogen on yield, harvest damage and stored potato quality of russet potato cultivars (Russet Norkotah CO8, Canela Russet, Rio Grande Russet with Centennial Russet in 2010 and Mesa Russet in 2011). These cultivars are among the most commonly grown russet cultivars in southern Colorado and the western United States, accounting for more than 70% of acreage (40,000 acres) in Colorado (NASS 2009). Previous research examining the relationship between tuber maturity and nitrogen fertilization has focused on issues that are of great importance to potatoes grown for processing, such as tuber specific gravity. The central focus of this study was to determine whether fresh market potato growers had higher returns when additional late season nitrogen was applied or when growers avoided a late season application.
1. May-August 2010: Conduct literature review and research planning. Plant field trials of four different cultivars at SLVRC and apply treatment rates of nitrogen. Prepare spreadsheets and organize for experimental harvest.
2. September-November 2010: Evaluate tuber maturity, yield and harvest damage and prepare samples for storage experiments to determine shrink and pressure bruising. Conduct initial analysis of yield data, harvest damage data, and moisture loss data and cost and benefit analysis.
3. February-April 2011: Evaluate shrink and pressure bruise from storage experiments. Analyze results and present at grower meetings. Evaluate methodology and results to determine potential improvements for second year research. Prepare seed and land for second year field trials.
4. May-August 2011: Plant second year field trials for four varieties at SLVRC and apply treatment rates of nitrogen. Prepare spreadsheets and organize for experiment harvest.
5. September-November 2011: Evaluate yield and harvest damage and establish second year storage experiments for determining shrink and pressure bruising. Conduct initial analysis of yield data, harvest damage data and moisture loss data.
6. February-May 2012: Evaluate shrink and pressure bruise from storage experiments. Analyze results and present to grower meetings. Compare results across both years experiment data. Prepare submissions for peer reviewed journals and commercial publications. Send surveys to growers on changes made to fertilizer regime and adoptions of research results.
All research plot trials were established at the San Luis Valley Research Center of the Colorado State University system. The experimental design used was a randomized block design to attempt to minimize field location effects on yield. Sixty pounds per acre of nitrogen were applied at planting, and three additional applications of about twenty pounds each were made during June and early July. For the first field research year, research plot trials were established for four russet varieties: Canela Russet, Centennial Russet, Rio Grande and Russet Norkotah CO8. Plots were planted on May 6, 2010 and late applied nitrogen treatments (control, additional 20 lbs. nitrogen, and additional 40lbs. nitrogen) were applied as a liquid foliar application on August 12. This was approximately 25 days prior to vine kill. Petiole samples were collected a week prior to treatment, as well as one week and two weeks following treatment. Research plots were mechanically harvested on September 28 and October 2. Tubers from each plot were weighed to determine yield and were evaluated for harvest damage such as skin peeling, shatter bruise and russet netting loss. Subsamples were also created following grading to be tested for tuber skin resistance to shearing force, specific gravity and moisture loss from 24 hours in a drying oven at 100 degrees Fahrenheit. Samples for long-term moisture loss and pressure flattening were placed in an experimental design for inducing pressure flattening (Castleberry and Jayanty, 2012). In the fall of 2010, because of limited space at the storage bins, samples were delayed from appropriate storage until late October. In 2011, samples were able to be placed in storage immediately after harvest and sampling in early October. After six months of storage in the container experiments the subsamples of tubers were weighed again and observed for the number and diameter of pressure bruises. This data allowed for determination of shrink loss from the different treatments and susceptibility to pressure bruising. In February 2011, a survey was developed and distributed at the 2011 Southern Rocky Mountain Agricultural Conference. The survey confirmed that roughly 25% of potato growers applied nitrogen in early August, and an additional 10% of growers had applied nitrogen in mid-August or later. This confirmed our hypothesis that local potato farmers were applying nitrogen late in the growing season. The most common reason for the late applications was to increase yield. The results also confirmed that three of the four cultivars that were selected for preliminary work in 2010 were the most important to the growers surveyed. Based on grower input, Centennial was replaced in 2011 research with the newer cultivar Mesa Russet. In 2011, plot trials were established, treated with nitrogen and harvested similar to the 2010 field experiments. Plots were harvested three weeks after defoliation of the crop. Yield and relevant at-harvest tests including skin set and specific gravity were conducted. Data from the at-harvest tests was processed and analyzed. Subsamples from the 2011 treatments were prepared for long term storage evaluation to determine pressure bruise development and stored for six months. After evaluation of the subsamples for shrink and pressure bruising, storage data was analyzed. All results were processed into spreadsheets and statistical analysis was conducted using a single factor analysis of variance using the Microsoft Excel 2007 data analysis toolkit software.
In 2010, Norkotah yield by size category results indicated significantly higher yields of potatoes less than four ounces or between four and six ounces for plants that received the additional nitrogen treatments (Table 1 and Figure 1). However, yield of larger tubers that were 12 to 15 ounces were significantly higher for the no additional nitrogen control treatment compared to the additional nitrogen treatments. This may reflect the plants with additional nitrogen being pushed towards increasing vine growth rather than continuing a steady rate of tuber bulking. There were no statistically significant results for the 2010 yield by size category data for Canela Russet(Table 1 and Figure 2). There is some evidence of a trend similar to that of the Norkotah yield in which yield of larger tubers is higher for the control treatment while additional late nitrogen only increased the yield of small and undersize tubers. Rio Grande Russet also had no statistically significant results within the 2010 yield by size category results (Table 1 and Figure 3). There were no obvious trends in the Rio Grande yield data. Data for the cultivar Centennial was discarded due to variability from poor stand establishment.
Results from 2011 yield trials indicated higher total yield than what was seen in 2010. For Russet Norkotah CO8 (Table 2 and Figure 4), there was a significant increase in yield of 12-18 oz. tubers for the control and the additional 20 pounds treatment compared to the additional 40 pounds nitrogen treatment. There was also a general trend towards increased yield of larger tubers with the additional 20 pounds treatment. This may have reflected a crop that was too low on nitrogen during the last few weeks of tuber bulking. These two years of data illustrate a difficulty with some Norkotah selections, and perhaps with some other cultivars, where a smaller additional nitrogen application may prolong bulking some years (2011) and in other years will actually slow or even halt yield development (2010). In 2011, Canela Russet produced significantly higher yields of 6 to 9 and 12 to 18 ounce tubers with 20 pounds of additional late nitrogen compared to both the control and the additional 40 pounds treatment (Table 2 and Figure 5). There was also some trends in the data indicating that there were higher yields of midsize and large tubers for the no additional nitrogen control compared to the additional 40 pounds treatment. The explanation may be that, unlike in 2010, the additional 20 pounds of nitrogen were useful for additional yield development. However the decrease in yield of larger tubers for the 40 pound treatment shows the potential of too much late season nitrogen to actually reduce yield.
There were no statistically significant differences in yield within size categories for either Rio Grande Russet (Table 3 and Figure 6) or Mesa Russet (Table 3 and Figure 7). In 2011, there appeared to be no trends or differences in yield for Rio Grande whether late nitrogen was added or not. Although plot to plot yield results were variable, there did appear to be a strong trend for increased yield of Mesa Russet in response to late fertilization, especially at the higher 40 pounds additional rate. This may indicate that Mesa Russet was deficient in nitrogen at the time of treatment or that cultivar specific factors may allow Mesa Russet to respond to higher late nitrogen by tuber bulking rather than reducing bulking in favor of renewed vine growth.
The soil at the San Luis Valley Research Center includes many rocks that are 6-10 ounces and many rough and jagged rocks. As a result, very high percentages of at harvest skin damage were observed. A typical farm would not usually experience as much damage as was observed in this study. In 2010, there was a trend towards increased skin removal and damage of tubers at harvest across all cultivars as a result of additional late season nitrogen, although the numbers were not statistically significant (Figure 8). Results from 2011 were less consistent (Figure 9), with trends for Canela Russet indicating increased damage with additional nitrogen but results for Rio Grande russet indicating an opposite trend. Because of the very wide variability in damage results, economic analysis presented in this report is mostly focused on yield and the returns for different size categories of potatoes. A modified torque meter designed to measure shear force resistance of skin was also used to evaluate the effect of late nitrogen applications on resistance to skin removal (Figures 10 and 11). The higher values indicate increased resistance and therefore more durable and more matured potato skin. A value of three or so would indicate moderate or susceptibility to skin damage, while four and above would indicate very mature and durable skin. There were no statistically significant differences (or strong trends) within each cultivar as a result of late nitrogen treatment. It is interesting to note that skin durability was higher in this test in 2011 compared to 2010.
Tuber specific gravity, which can also provide an indication of tuber maturity, was tested at harvest. In general, higher specific gravities indicate more crop maturity within a specific cultivar. The 2010 results were varied across the cultivars, with higher specific gravities observed for the 20 lbs additional nitrogen treatment for Russet Norkotah CO8 and Canela Russet, while there was a trend towards decreased specific gravity with additional nitrogen for Rio Grande Russet.
An important part of the research program was to evaluate if there were storage problems that result from late season nitrogen. There were no observed differences in tuber moisture loss (shrink) during storage (data not shown). When pressure flattening was measured after six months of storage, no statistically significant differences were observed in 2010 and 2011 (Figures 14 and 15). There were, however, some interesting trends that may indicate a small reduction in pressure flattening as a result of the 40 pounds additional nitrogen treatment. It is unclear why heavy applications of late nitrogen would have this effect. Additional research may reveal whether this is a consistent effect.
Educational & Outreach Activities
The project and its goals were presented at the 2011 San Luis Valley Research Center Field Day and at less formal research center tours by industry groups and a College of Agriculture faculty tour of the research station. The project was also included as an example of industry orientated applied research in meetings held between the research center faculty and the state potato growers organization the Colorado Potato Administrative Committee (CPAC). At the 2012 Southern Rocky Mountain Agricultural Conference, we presented to approximately 120 farmers and industry representatives our initial results based on the research conducted in 2010- 2011. During the presentation, the growers were informed that the research was made funded by a research grant through Western SARE. Growers were also provided with contact information if they wanted to be on a list to receive the report directly, and we will use the registration records for the conference as a list to disseminate the final economic information. The direct comments received from a few farms after the conference have been that the project is one that is both very useful and applicable to their farms. One storage manager stated it to be one of the best projects presented at the conference in terms of their belief that it was an appropriate use of research funding. Organizers of the conference circulated a survey asking growers and industry about how useful each presentation at the conference was to their farm or production operations. The responding farmers rated the presentation “Effects of Late Nitrogen Applications on Profitability of Colorado Russet Cultivars” as 4.3 out of 5 with 1 being not useful at all and a 5 meaning extremely important to their farm.
Results from the project were also presented at the 2012 Potato Association of America conference in Denver, Colorado. Approximately 40 industry representatives and potato researchers attended the research presentation “Influence of Late Growing Season Nitrogen Application on the Tuber Yield, At-Harvest Quality, and Stored Potato Quality of Russet Potatoes.” The presentation was included in the session designated as “Industry-Orientated.”
Due to the relatively few statistically significant findings on yield and the inconsistent response of some cultivars to treatment from one year to the next, it is not likely that the results will be developed into a stand alone peer reviewed publication. However, some results are likely to be included as part of a journal publication on late season nitrogen applications that will be mostly based on three years of companion research funded through NRCS. It is our intent to disseminate a brochure based on the Western SARE funded graduate student grant research at the next Southern Rocky Mountain Agricultural Conference in 2013 and to publish the final economic analysis as a short article in the potato extension Spud Topics newsletter. In addition the research will be included as a portion of the graduate student’s final dissertation.
The measurable impacts of the project would be increased returns and quality to potato farms that grow the specific cultivars tested, and more awareness of the potential drawbacks of late nitrogen applications. One specific outcome could be improved returns from shipments of Canela Russet due to reduced late season fertilization. Previously, some growers have expressed disappointment in the storability of immature Canela Russet tubers. It would be expected that based on these experiences and the dissemination of this research through field day and conference presentations, that growers will no longer try to “push” Canela Russet yield with additional late season applied nitrogen. The two years of Canela yield data presented should convince growers that there is not likely to be an increase in yield from late fertilization for that cultivar. Additionally, growers of the CO8 and CO3 selections of the cultivar Russet Norkotah may reduce their late fertilization because of the findings of this research, which indicates no clear economic benefit from late applications.
A summary of the economic impacts of the research is based on records from the USDA Agricultural Marketing Service for Colorado shipping point prices for the first week of January in each year following the plot trials. Prices for US No. 1 10lb film bags were used for potatoes 4-9 ounces in size and US No. 1 Carton 70 prices for price of potatoes greater than 9 ounces. In general, prices for Carton 40s, Carton 50s, Carton 60s, etc were very close to Carton 70 prices at these shipping dates. A 10% overall shrink was assessed to account for normal storage loss, and this approximates AMS statistics on the statewide average loss between harvest and sale. It should be noted that it is assumed that the remaining 90% of tubers that are over four ounces are assumed to be US No. 1 quality. A loss of $0.50 per cwt. was also assessed for tubers less than four ounces to reflect the costs of harvesting, sorting and discarding these potatoes which are below acceptable size for most fresh potato markets but still likely to be harvested and make it to storage. Nitrogen costs were assessed at $0.60 per pound nitrogen based on a local delivered price of a 34-0-0 liquid nitrogen fertilizer at 2009-2010 prices. Labor and application costs were not estimated as it is likely that the fertilizer would be applied as part of a routinely scheduled irrigation application. Based on these assumptions, the gross returns for Norkotah with no additional late season nitrogen was $4,876 per acre in 2010 and $7,104 per acre in 2011. When 20 lbs. of additional nitrogen were used the crop value for Norkotah was $4,253 per acre in 2010 and $7,658 per acre in 2011. At the 40 lbs. of additional nitrogen rate, the Norkotahs were worth $3,807 in 2010 and $6,767 in 2011. So for both years returns were higher for the Norkotahs that did not have additional nitrogen added compared to the 40lb. additional nitrogen. In 2010, returns were higher for the no additional nitrogen compared to the 20 additional pounds but lower than the 20 additional pounds in 2011.
The gross returns for Canela Russet with no additional late season nitrogen were $5,764 per acre in 2010 and $7,033 per acre in 2011. When 20 lbs. of additional nitrogen were used the crop value for Canela was $5,345 in 2010 and $7,792 in 2011. At the 40 lbs. of additional nitrogen the Canela were worth $5,484 in 2010 and $5,958 in 2011. Similar to the Norkotah results, in both years gross returns were higher for the Canela Russet without additional nitrogen compared to the 40lb. additional nitrogen. Returns were higher in 2010 for the no additional nitrogen compared to the 20 additional pounds, but in 2011 they were lower than the 20 additional pounds treatment.
For Rio Grande Russet the gross returns for the no additional nitrogen treatment were $7,322 per acre in 2010 and $8,612 per acre in 2011. The 20lb. additional N treatment produced $7,573 in 2010 and $8,601 in 2011. Lastly, the 40 lb late nitrogen treatment had a gross return of $7,370 per acre in 2010 and $8,671 in 2011. These results indicate that the 2010 Rio Grande crop did benefit slightly from the 20 lbs. additional late nitrogen, but there was no difference in returns for the 40lb. treatment or a difference in returns in 2011 as a result of late season fertilization.
Mesa Russet, which was only studied during the 2011 season, did provide higher returns due to increased fertilization. The no additional nitrogen treatment produced gross returns of $7,014 per acre while 20lb. and 40lb. treatments returned $7,217 and $8,650 respectively. It should be noted that it is unclear if the returns would be similar if studied for a second year.
At this point we have only anecdotal information that a few area growers are less likely to apply additional nitrogen because of this research. Additional dissemination of the information at next year’s regional conference and field days may allow for a substantial post-research survey to measure changes in fertility management.
Representatives from many of the potato operations in Colorado were present at the 2012 Southern Rocky Mountain Agricultural Conference. Between both the field days and conferences, about 80-100 farm managers and agronomists have received at least the initial conclusions of this research. The primary recommendation from the research is to avoid late season nitrogen applications unless there is a clear deficiency for most russet potato cultivars. As evidenced by the yield and economic return analysis, some cultivars will lose money if late season nitrogen is applied and other cultivars will not provide any consistent improvement. Only Mesa Russet, for which we had only one year of data, had a strong positive response to additional late season nitrogen fertilization.
Areas needing additional study
Additional research may be required to determine at what petiole or total tissue concentrations a specific cultivar is nutrient deficient and could benefit from late nitrogen, and whether mid-season applications will be more productive than late season applications. Additional years of research may also help us conclude whether there is a connection between late season fertilizer applications and increased pressure bruising. The post-harvest research program at CSU is currently evaluating the effects of middle to late season applications of an organic nitrogen source to determine if organic growers receive benefits from relatively late nitrogen applications.