THE DESTINY MODEL IS ALSO SUITABLE FOR TMR PRODUCTION SYSTEMS

The DESTiny model measures greenhouse gas (GHG) emissions and biogenic carbon flows across the animal, feed, external input, soil and economic components of a dairy farm, breaking these down into a net GHG balance expressed per farm, per kg fat- and protein-corrected milk (FPCM), per livestock unit (LSU) and per hectare. Methane (CH4) and nitrous oxide (N2O) emissions are expressed as CO2-equivalent using IPCC (2023). The model has been extensively tested on pasture-based farms with great success, and some three scientific papers resulted. This report deals with results obtained on TMR farms.

The results of five herds were obtained, but because of the FMD outbreak and incomplete data on some farms, the results of two typical farms were simulated. Farms 1 and 2 were these theoretical scenarios representing a large-framed Holstein-type herd (Farm 1, 710 kg mature body weight) and a small-framed Jersey-type herd (Farm 2, 425 kg mature body weight). Farm 4 was a real operation but had limited input data in several sections – it grows no feed, and some of its purchased feed and economic entries were incomplete or estimated. Farms 3 and 5 were the most detailed datasets, with full animal, feed and (for Farm 5) economic information; Farm 3’s economics were less complete than Farm 5’s because its overhead costs were not fully recorded. Where values were missing, they were estimated based on the more complete farm data. The dairy herds varied significantly in size, cow weight and production levels. Herd sizes ranged from 300 to 2,400 productive cows (about 393–4,588 LSU). The mature live weights ranged roughly from 425 to 750 kg, indicating a mix of medium- and large-framed cows. The average milk yield spanned from 16.5 to 42.3 kg per cow per day, supported by cow dry-matter intakes of about 17.6 to 28.1 kg DM per cow per day from the TMR diets. The farms produced between 1.8 and 30.6 million kg of FPCM annually. The diets of cows in lactation were relatively high in energy with crude protein contents of 13–18% of DM, 18–27% ADF, 32–45% NDF, and around 6% dietary fat. Milk components ranged from 3.6 to 5.1% butterfat and 3.2 to 4.0% true protein. Feed efficiency, measured as kg FPCM per kg DMI, varied from approximately 1.10 to 1.47, indicating notable differences in how effectively each herd converted feed into saleable milk solids under the respective TMR management systems.

The summarised results are: Across the five TMR herds, DESTiny calculated three net GHG sources (Farms 1, 2 and 4) and two net sinks (Farms 3 and 5). Enteric CH4 was the dominant positive emission on every farm, but the biogenic sink term, driven by on-farm biomass accumulation and purchased-feed carbon, is what separates sources from sinks. Farm 5 was the largest sink in absolute terms (−11,516 t CO2e/year; −0.38 kg/kg FPCM) on the strength of its large forage base (1,494 ha, 260 ha irrigated, 21,945 t DM of home-grown feed) and high milk output, while Farm 3 had the deepest intensity per unit milk (−1.11 kg/kg FPCM) through high per-hectare biomass accumulation relative to a small milk output. Farm 4 was the largest source (16,225 t CO2e/year) because its fully purchased ration generates high external-input and animal emissions with no on-farm biomass to offset these. Manure CH4 is elevated on Farm 5 (28% of total CH4) owing to its lagoon system and high volatile-solids output, and on Farm 3 (12%) through its larger slurry fraction. The nitrogen balance was internally consistent, with excretion rates of 69–71% across herds, confirming that the model partitions feed N sensibly. Economically, Farm 5 was the most profitable (R130.8 million profit, 54% margin) and Farm 3 the least (R1.8 million, 12%); among the herds with estimated costs, Farms 2 and 4 fell in a realistic 13–20% range and Farm 1 was approximately break-even under full-cost allocation.

In conclusion: DESTiny can clearly fully depict TMR systems, not just the animal and manure emissions that most carbon-footprint tools focus on, but the entire biogenic carbon ledger of the feed base (including home-grown and purchased biomass, biological N fixation, and animal respiration), which ultimately determines whether a farm is a net source or a net sink. This capability explains how Farms 3 and 5 could be accurately identified as true net sinks rather than just lower-emission sources, a distinction that conventional partial-inventory methods cannot do. The results are also consistent with the earlier work on pasture-based systems showing that improved forage bases and nutrient management can turn some South African dairy operations into genuine net carbon sinks.