“Dairy forages – corn, sorghum, alfalfa, fall-planted cereals – are grown on approximately 1.6 million acres per year in California,” said Nick Clark, farm adviser in Kings, Tulare and Fresno counties. “They support a $7-billion-a-year milk production industry.

“This very high-value commodity is facing the need and an opportunity for improvements in input efficiency,” he continued.

Forage crop production is often performed on fields with low-tech irrigation. Manure applied as fertilizer also tends to be done relatively imprecisely compared to synthetic fertilizer application processes.

Speaking at the spring Golden State Dairy Management Conference in Modesto, California, Clark said his recent two-year study evaluated many things in addition to field nitrogen balance. Some other areas studied were other macronutrient fertility and water-use efficiency.

Beyond efficient crop production considerations, dairy operators are also under regulatory pressure to improve nitrogen-use efficiency with manure fertilizer applications to crop fields. The Regional Water Quality Control Board issues and re-issues general orders describing monitoring and reporting requirements and waste discharge requirements with the purpose of mitigating groundwater pollution by nitrate from dairy manure land applications. The Central Valley Dairy Representative Monitoring Program (CVDRMP) has conducted extensive groundwater quality trend monitoring relating dairy manure-handling practices to groundwater quality.

“Each plot (the entire field scale evaluations took place on just over 30 acres) was the size of an irrigation set, or ‘check,’ to capture the field scale variability inherent in dairy forage production systems,” he said. “We compared withholding nitrogen from crops to nitrogen from manure applications only to nitrogen from manure plus synthetic nitrogen fertilizer. The latter treatment was considered the cooperating grower’s standard practice. We compared the total amount of nitrogen applied from each measured source to the amount of nitrogen removed in the harvested crops. The water board regulates that applied nitrogen cannot exceed 140 percent of nitrogen removed in harvested plant parts.”

In the first year of corn, Clark and his team did not observe remarkable yield differences between treatments, though they were statistically different. The “zero nitrogen control” yielded significantly less than the “manure only” treatment.

They did the same comparison of nitrogen applied versus removed in harvested plant parts with the following winter wheat crop. The grower applied significantly larger amounts of manure before the wheat crop was planted compared to the previous corn crop. The applied nitrogen versus removed nitrogen ratios for each treatment were proportionally much higher than for the previous corn crop.

“With the amount of manure applied to the manure treatments, we didn’t see any benefit to yield by applying synthetic nitrogen fertilizer as well,” Clark said. “We did see a statistical and remarkable yield penalty for not applying nitrogen to the wheat – up to a 5-ton-per-acre yield loss.”

Other observed disadvantages in-season were a weaker stand of wheat, which allowed the rapid, widespread establishment of weeds; and very early termination of the crop, which dried out faster than desirable for making quality silage. The slower mineralization of nitrogen in the colder winter months might have played a strong role in reducing nitrogen availability to the plants in the “zero control” treatment, making nitrogen stress much more apparent than in the previous corn season.

After seeing the amount of manure nitrogen applied in the previous wheat crop, the grower opted to withhold manure application from the corn crop. The fact the crops were still able to take up more nitrogen than was applied in that season is testament to the slowly occurring, long-term plant availability of nitrogen as it’s released from the organic nitrogen pool in soil that has been repeatedly amended with manure. This study did not evaluate the long-term effects of withholding manure.

“Yield was not statistically different between nitrogen treatments in corn,” he said. “We think this is testament to the stored organic nitrogen from previous manure applications becoming quickly plant-available during the warmer spring and summer months. We do not know how long this benefit to summer crop yield could be expected to last, but we did not see great negative impacts to corn yield for two years.”

Dairy manure sources generally have proportionally more potassium and phosphorus compared to nitrogen than the corn crop removes from the soil as it grows. This means if manure is treated like a nitrogen fertilizer, and fertilizing corn based on its nitrogen need, farmers are likely to overapply potassium and phosphorus.

“An alternative approach would be to treat manure like a potassium fertilizer and fertilize corn on the basis of its potassium need,” Clark said. “This might under-fertilize with nitrogen, and the balance of crop need may need to be met with applications of synthetic nitrogen fertilizer.”

Two main challenges with this approach are unresolved. First, it is very difficult, if not impossible, to accurately and quickly measure the amount of nutrients being applied to the crop via manure. Second, making precise synthetic nitrogen fertilizer applications in-season to the corn crop requires additional training and labor to evaluate the soil and plant tissue to determine crop need in real time. The first can be resolved with research. The second can be resolved by training with existing resources.

“Alfalfa is a high consumer of phosphorus and potassium, so manure is a good fertilizer to use,” he said. If acres to land apply manure are limited on-farm, considering alfalfa fields as a potential location to apply manure can expand those available acres and bring the whole dairy farm closer to an agronomic balance of nitrogen applied versus removed in harvested plant parts.”

Alfalfa has several conceivable benefits to being grown on cropland that has been repeatedly amended with manure. Improved weed management may be one of the greatest, as with alfalfa being planted in rotation with corn and wheat, herbicides selective for grassy weeds which may have been building up in corn/wheat double cropping become more available for use. In soils with a lot of plant-available nitrogen, alfalfa will preferentially take up that nitrogen before fixing its own.

“There are several guidelines to avoid some common pitfalls associated with applying manure to alfalfa cropland: introducing more weed seeds and causing ammonia or salt injury to the crop,” he said. “Research results show no nutrient removal difference between alfalfa fertilized with manure versus fertilized without manure.”  

PHOTO: California farm adviser Nick Clark said he hopes dairy farmers, crop consultants and environmental labs understand how complicated finding the right nitrogen balance can be. Photo provided by Nick Clark.

Julia Hollister is a freelance writer based in San Francisco, California.