A study from 2011 and 2012 up in Saskatchewan looked at how the rate and uniformity of solid cattle manure application impacted crop yield under two fertilizer practices, manure application only and manure with supplemental urea fertilizer. This study is unique because it focuses on solid manure application uniformity with and without supplemental fertilizer. It used three different manure spreaders to give different manure application uniformities.
Manure rates tested were 0, 9, and 27 tons per acre, respectively, with three different coefficients of variation of manure application achieved (10, 50, and 110%). The low application rate supplied approximately 150 lb total N/acre, while the high rate supplied around 450 lb total N/acre. No statement on the fraction estimated to be plant available was provided, but it is fair to assume approximately 50%. On half of the plots, an additional 71 lb N/acre was applied using urea. They repeated two years growing oats in year one and barely in year two. Each year's yield was scaled to the percent of max for that year and then averaged.
Figure 1. Illustration of how solid cattle manure application rate (tons/acre) and uniformity (coefficient of variation) impacted crop yield (for oats and barley) as a function of maximum yield achieved.
Important notes and take-homes:
In all cases, the use of urea increased crop production, causing a significant increase (p = 0.0005) in yield. Solid manure can provide sufficient nitrogen to support crop nitrogen need on an annual basis. Still, in many cases, the higher carbon content of the manure causes early-season nitrogen tie-up, so while overall availability may be sufficient, there could still be periods of inadequate nitrogen supply.
There were no statistical differences among treatments where urea was applied. These treatments all had sufficient nutrients to maximize yield. Urea on its own was sufficient to provide the nitrogen the crops needed.
There was no statistical difference between manure application rates (p = 0.22). While we think more is often better, adding more manure didn't improve yield, presumably because while overall nitrogen supply was increased, a greater tie-up of nitrogen occurred with the greater manure application rates, causing a more seasonal deficiency.
While no impact of application variability was seen on the urea applied plots, in the manure-only plots, application variability of 50% had more significant yields than 10% statistically but was no different than 110%. Generally, the data suggest improved uniformity increased yield at the higher application rate, where manure had to supply fertility.