（一）产量的定义

Agronomic Yields

1. Abstrct

Agronomic yields are defined as the crop grain and non-grain biomass harvested in a given growing cycle and serve as the basic measurement for comparing production differences among treatments.

For annual row-cropping systems of corn, soybean and wheat, yield is determined as the grain biomass collected during harvest; for annual sorghum, yield is determined as the non-grain biomass harvested.

For perennial crops, including monoculture systems, such as switchgrass and miscanthus, and polyculture systems that are harvested annually, such as restored prairie, native grasses, early successional (GLBRC only) and historical vegetation (GLBRC only), yield is determined as the biomass harvested at the end of the growing season.

For alfalfa, harvested several times per year, yield is the sum of biomass harvested over the growing season. For woody cropping systems of short-rotation poplar, annual yield is determined as the biomass collected at clear-cutting divided by the number of years of growth.

2. Protocol

2.1 Grain Harvest of Annual Row-crops

Grain yields of annual row-crops (corn, soybean and wheat) are measured using two different procedures:

(1) harvested grain is off-loaded into weigh a wagon (equipped with load cells) and weighed on-site. Yield reported in bushels per acre (bu/ac) at standard moisture content (see below).

(2) grain harvesters are fitted with a GPS-referenced yield monitoring system that provides yield maps for each 1 ha treatment plot. See the precision agriculture yield monitoring dataset (KBS037) for more information.

US yields of annual grain crops are measured in bushels (bu), which are units of mass that vary among crops.

标准含水量 ： For corn, the standard employed is 56 pounds per bushel at 15.5% moisture;for soybean and wheat it is 60 pounds per bushel at 13% moisture

干重： The dry weight of a bushel of corn is 47.32 pounds; that of soybean and wheat is 52.2 pounds.

Yields are recorded in bushel per acre and reported in kilogram per hectare at standard moisture (kg/ha).

Since 2004, wheat straw is baled after grain harvest and removed from MCSE treatments T1 and T2 and is chopped and left on the field in T3 and T4. Prior to then, the straw was typically baled and removed from all treatments. For an estimate of wheat stover for the MCSE, use data from annual net primary production sampling (see datatable KBS019-003) and subtract seed biomass from the whole plant biomass.

2.2 Biomass Harvest of Herbaceous Crops

Alfalfa (MCSE T6, retired in 2019 and planted to switchgrass) was typically cut (mowed) three times per growing season. Cuttings were raked into windrows and either baled, loaded onto a wagon, or chopped, and weighed at the Kellogg Farm. Grab samples from each replicate plot were placed in paper bags, weighed, dried, and re-weighed to determine the percent moisture in the harvested biomass for each plot; the moisture content is subtracted from the harvested fresh weight to produce a dry weight estimate of the total alfalfa harvested. Alfalfa yield is reported as metric tons of dry biomass per hectare (MT/ha).

Herbaceous monoculture (switchgrass, miscanthus, sorghum) and polyculture systems (native grasses, restored prairie, early successional (GLBRC only) and historical vegetation) are typically harvested at the end of the growing season in one of three different ways, depending on size of plot (see the Ag log for experiment specific procedures). All methods leave a 4-6 inch stubble of remaining vegetation. In narrow microplots, fresh weight yields are measured in a predetermined 7.6’ wide section of the plot with an on-board biomass harvester weighing system. In larger experimental subplots or split plots, the biomass of either a predetermined section or of the entire plot is harvested with a forage harvester, chopped into a forage wagon equipped with load cells, and weighed on-site. In field-scale plots, the entire field is harvested and windrowed, and biomass is baled. Bales are counted and initially every bale was loaded onto a wagon and weighed at the Kellogg Farm or a subsample of bales was weighed to determine an average fresh weight per bale. With the addition of a tail-gate scale, each bale is now weighed and recorded individually as the bale emerges from the baler. Grab samples from each replicate are placed in paper bags, weighed, dried, and re-weighed to determine the percent moisture in the harvested biomass; the moisture content is subtracted from the harvested fresh weight to produce a dry weight estimate of the total harvest. Yields are reported as megagrams dry biomass per hectare (Mg/ha).

2.3 Biomass Harvest of Woody Crops

Agronomic yields for poplar treatments are determined at stand clearcutting, which occurs approximately 8-10 years after planting or coppicing and during winter months. Yield is either a measure of the weight of all trees in the replicate plot (1998 and 2008 for the MCSE) or a measure of the average tree weight based on the biomass of randomly selected harvested trees (2018 for the MCSE and GLBRC). Consult the Ag log for specific details. To determine the dry weight of harvested trees, subsamples of chipped trees are weighed fresh, dried in an oven at 60° C, weighed again, and a fresh weight to dry weight conversion factor determined for whole trees. Dried biomass per tree is converted to an areal basis using the following planting densities:

• 1989 MCSE: 1 tree per 2 m2
• 2008 MCSE: 1 tree per 3.6 m2
• 2008 GLBRC: 1 tree per 3.6 m2

3. Calculation

将bushels per acre (bu/ac)换算为 Per hectare (kg/ha)

1. For grain of annual crops:

• To convert bushels of corn per acre (bu/ac) to kilograms corn per hectare (kg/ha), first express bushels as a weight in kilograms (bushels x 56 lbs/bu x 0.4536 kg/lb) then divide by 0.4047 ha/ac. To convert from standard moisture to dry weight, multiply yield in kg/ha by 0.845 (=1-0.155).

换算公式： Per hectare (kg/ha) = bushels per acre (bu/ac) * ( 56 lbs/bu x 0.4536 kg/lb) / 0.4047 ha/ac * (1-0.155)

• To convert bushels of soybean or wheat per acre (bu/ac) to kilograms soybean or wheat per hectare (kg/ha), first express bushels as weight in kilograms (bushels x 60 lbs/bu x 0.4536 kg/lb) then divide by 0.4047 ha/ac. To convert from standard moisture to dry weight, multiply yield in kg/ha by 0.87 (=1-0.13).

换算公式： Per hectare (kg/ha) = bushels per acre (bu/ac) * ( 60 lbs/bu x 0.4536 kg/lb) / 0.4047 ha/ac * (1-0.13)

• For all crops, then calculate the mean yield and standard error for the treatment (n = number of replicates).

2. For herbaceous crops:

• For alfalfa first determine the annual alfalfa yield per replicate as the sum of all harvests in a given year.

3. For all crops, then calculate the mean yield and standard error for the treatment (n = number of replicates) For harvested poplar:

• To determine annual poplar yield, divide the harvested biomass per replicate by the number of years of growth then calculate the mean annual poplar yield and standard error for the treatment (n= number of replicates).

（二）换算公式

将生物量(g/m2)换算为 bushels per acre (bu/ac) ，通常用于模型模拟结果与观测产量比较

For each year, the USDA NASS reports the average yields for various crops at the county‐level over the United States (https://quickstats.nass.usda.gov/). These data are based on harvested yields, reported by a sample of farmers within each county, and verified with independent yield samples taken by USDA staff when the crop reaches maturity (FAO and DWFI, 2015). Therefore, the model simulated biomass (g/m2) will need to be converted to standard yield (bushel/acre,bu/ac) to compare with the USDA county‐level data, following the instruction (see http://www.ag.ndsu.edu/pubs/plantsci/crops/ae905w.htm):

In Equations 1 and 2, 0.155 and 0.13 are the standard moisture content (15.5% and 13%) for corn and soybean, respectively.

Harvested corn usually contain an initial moisture content greater than 15.5% (15.5~32%). For transportation and storage purpose, mechanical drying method is typically applied to reduce the initial moisture to the standard moisture.

Two sources of weight loss are associated with this process: (1) the weight of the moisture loss (also known as “water shrink”) and (2) the weight loss due to handling processes (Hicks & Cloud, 1992). The handling loss could range from 0.04% to 5.22%, depending on the initial moisture content and shrinkage loss.

Therefore, the calculated dry mass losses tend to be variable among different growers. This uncertainty is worth noting when comparing the model simulated dry mass with standard yield in the USDA survey.

（三）一些补充

• one metric ton (MT) = 1 Megagram (Mg) = 1,000 kilograms (kg)=1000,000 (g)

• a hectare = 2.471 acres =10000 m2

• 中文含义
  metric ton ：公吨（简写为吨）
  Megagram：兆克
  bushel： 蒲式耳（容积单位）
  hectare： 公顷
  acres： 英亩

1. Grain yields

Some of our traditional terms are more complicated to convert from one system to another. （1）Grain yields for example are conventionally expressed in terms of volume per acre (bu/a). In the metric system,（2） yield is given by weight (kilograms per hectare).

玉米产量： A corn yield of 200 bushels per acre is first expressed by weight (200 bu * 56 lb/bu = 11,200 lbs) and then converted to kilograms (11,200 lbs * 0.4536 kg/lb = 5,080 kg). Because a hectare is equal to 2.471 acres, it means that 200 bu/ac is equal to about 12,553 kg/ha (5,080 kg/ac x 2.471 ac/ha = 12,553 kg/ha). This translates into 126 quintals per hectare and 12.55 metric tons per hectare.

即： 200 bu/ac = 12,553 kg/ha (5,080 kg/ac x 2.471 ac/ha = 12,553 kg/ha)

2. Corn (56# bu)

• 1 kilogram/hectare (kg/ha) = .0159 (.016) bushels/acre

• 1 bushel/acre = 62.77 (63) kilograms/ hectare

• 1 quintal/hectare (q/ha) = 1.593 (1.6) bushels/acre

• 1 bushel/acre = .6277 (.63) quintals/hectare

• 1 metric ton/hectare (MT/ha) = 15.93 (16)
  bushels/acre

• 1 bushel/acre = .0628 (.06) metric tons/hectare

3. Wheat/soybeans (60# bu)

• 1 kilogram/hectare (kg/ha) = .0149 (.015) bushels/acre

• 1 bushel/acre = 67.25 (67) kilograms/hectare

• 1 quintal/hectare = 1.487 (1.5) bushels/acre

• 1 bushel/acre = .6725 (.67) quintals/hectare

• 1 metric ton/hectare = 14.87 (15) bushels/acre

• 1 bushel/acre = .0673 (.07) metric tons/hectare

（四）参考文献

【1】: 产量的定义
【2】: 换算公式
【3】: 一些补充
【3】: 含水量计算讨论 research gate