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. 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. 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.

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).

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
• 2009 MCSE: 1 tree per 3.6 m2
• 2019 MCSE: 1 tree per 3.6 m2 (trees are planted on a 2.43 × 1.52 m spacing)
• 2008 GLBRC: 1 tree per 3.6 m2 ((trees are planted on a 2.43 × 1.52 m spacing)
• 2019 GLBRC: 1 tree per 3.6 m2 (trees are planted on a 2.43 × 1.52 m spacing)

Calculations

Note that one metric ton (MT) = 1 Megagram (Mg) = 1,000 kilograms (kg)

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).
• 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).
• 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).