Agriculture Reference
In-Depth Information
the rest (IPNI 2012). In North America, most synthetic fertilizer N is applied
as anhydrous ammonia (27% of inputs), urea ammonium nitrate (24%), or urea
(23%) (IFA 2011).
Synthetic fertilizers used in the MCSE include different combinations of ammo-
nium nitrate (applied in N-P-K formulations) and urea ammonium nitrate (UAN)
(Table 9.3). Rates of N application in the Conventional and No-till systems have
ranged from 112-163 kg N ha
−1
yr
−1
for corn (
Zea mays
L.) and from 56-90 kg N
ha
−1
yr
−1
for winter wheat. The Reduced Input system receives ~one-third of the
synthetic inputs of the Conventional system (Table 9.3). No synthetic N fertilizer
is applied to the Biologically Based system or to soybean and Alfalfa (
Medicago
sativa
L.), and no system receives manure or other organic N forms. Poplars receive
between ~120 to 160 kg N ha
−1
at or shortly after planting.
Decisions about N fertilizer rates in MCSE annual crops are guided by MSU
Extension recommendations as well as past practice and best judgment. Prior
to 2008, Extension recommendations for the Conventional and No-till corn and
wheat rotations were based on the yield goal approach (Warncke et al. 2004),
which calculates base rates from past yields and projected yield increases. Since
2008 recommendations for corn have been based on the Maximum Return to
Nitrogen approach (MRTN; Warncke et al. 2009), now used by seven U.S. Corn
Belt states including Michigan (ISU 2004). For MRTN, N is applied on the basis
of statewide N response trials weighted by the price of fertilizer and corn to pro-
vide an economically optimized N rate. In 2011 we applied 156 kg N ha
−1
to the
Conventional and No-till corn in the MCSE. This is very close to the Economic
Optimum Nitrogen Rate (EONR) of 155 kg N ha
−1
based on 5-year average
corn yields for different N-fertilizer levels in the adjacent Resource Gradient
Experiment (Fig. 9.2; Robertson and Hamilton 2015, Chapter 1 in this volume).
For wheat, a yield goal approach still guides N rate recommendations; in 2010
we applied 89 kg N ha
−1
to wheat, which exceeded the EONR rate of 68 kg
N ha
−1
based on the average yields (2007 and 2010) for wheat in the Resource
Gradient Experiment (Fig. 9.2). The EONR is always less than the agronomic
maximum nitrogen rate, the rate at which agronomic yields are maximized (Fig
9.2), because at some point the cost of additional N fertilizer is greater than the
income provided by more yield.
Over the 1993 to 2010 period, corn in the Conventional and No-till systems
received 141 kg N ha
−1
yr
−1
, on average. This value is close to the statewide and
national averages of 134 and 149 kg N ha
−1
yr
−1
for the same period (NASS 2014).
For wheat over this period, we added 76 kg N ha
−1
yr
−1
, on average, ~29% lower
than the average N rate applied to wheat in Michigan (107 kg N ha
−1
yr
−1
) between
2004 and 2009, but very close to the national average (75 kg N ha
−1
yr
−1
) for the
same period (NASS 2014).
Nitrogen Fixation
Approximately 78% of the atmosphere is composed of dinitrogen gas (N
2
), which
is unusable by most organisms because of the strong triple bond between two
