Crop residues module

author: young

At the end of the growing season AFO has the option of harvesting or baling each crop, which leaves stubble for stock consumption, or crops can be left standing for fodder grazing. Stubble and fodder are modelled in the same ways, as follows.

Stubble and fodder are a key feed source for sheep during the summer months. In general, sheep graze crop residues selectively, preferring the higher quality components. Thus, they tend to eat grain first, followed by leaf and finally stem. To allow the optimisation of the quantity of the stubble grazed and to reflect selective grazing the total crop residues are divided into ten categories. The higher categories are better quality but generally lower quantity. Consumption of a higher quality category allows the consumption of a lower category (e.g. sheep can not consume any of category B until some of category A has been consumed).

The total mass of crop residues at first grazing (harvest for stubble and an inputted date for fodder) is calculated as a product of the biomass, harvest index and proportion harvested (see f_biomass2residue). Over time if the feed is not consumed it deteriorates in quality and quantity due to adverse effects of weather and the impact of sheep trampling.

Residue production can be positively impacted by frost because frost during the plants flowering stage can damage cell tissue and reduce grain fill [ZCC+15]. This results in less grain and more residue due to not using energy resources to fill grain. Thus, the harvest index used to calculate biomass to residue is adjusted by a frost factor. The frost factor can be customised for each crop which is required because different crops flower at different times, changing the impact and probability of frost biomass reduction. Frost factor can be customised for each LMU because frost effects can be altered by the LMU topography and soil type. For example, sandy soils are more affected by frost because the lower moisture holding capacity reduces the heat buffering from the soil.

To represent crop residues in AFO requires the proportion of total residue in each category and the DMD (quality) of each category. The DMD of each category is an input which a proportion of the total residue is allocated to. The proportion in each category was determined using AFO’s residue simulator which leverages the AFO stock generator (documented in a future section) in combination with trial liveweight data (Riggall 2017 pers comm). Using AFO’s stock generator, animals that reflect those in the paddock trial were simulated on large range of diet qualities and daily intake, and liveweight change was determined. The liveweight change of the simulated animals was compared with the actual liveweight change in the paddock trial to determine the daily feed quality. Based on the number of sheep, the sheep intake, and the total crop residue available in the trial, the proportion of residue in each category was calculated.

The energy provided from consuming each crop residue category is calculated from DMD. Like pasture, crop residue FOO is expressed in units of dry matter (excluding moisture), therefore feed energy is expressed as M/D (does not require dry matter content conversion). The volume of each crop residue category is calculated based on both the quality and availability of the feed.

Farmer often rake and burn crop residue in preparation for the following seeding. This is represented as a cost see Phase.py for further information.

Stubble grazing optimisation in AFO includes:

  • The time to start grazing of each stubble

  • The class of stock that grazes the stubble

  • The duration of grazing

  • The amount of supplementary required in addition to stubble (to meet alternative LW profiles)

Stubble definitions:

  • Total Grain = HI * (above ground) biomass

  • Leaf + Stem = (1-HI) * biomass

  • Harvested grain = (1 - spilt%) * Total grain

  • Spilt grain = spilt% * Total grain

  • Stubble = Leaf + Stem + Spilt grain

  • Spilt grain as a proportion of the stubble = (HI * spilt %) / (1 - HI(1 - spilt%))

AfoLogic.CropResidue.crop_residue_all(params, r_vals, nv, cat_propn_s1_ks2)

Calculates the crop residue available, MD provided, volume required and the proportion of the way through the feed period that crop residue becomes available.

AfoLogic.CropResidue.f_biomass2residue(residuesim=False)

Residue produced (Stubble or standing fodder) per kg of biomass .

This is a separate function because it is used in residue simulator.