Potential to reduce greenhouse gas emissions through different dairy cattle systems in subtropical regions

Abstract

Carbon (C) footprint of dairy production, expressed in kg C dioxide (CO2) equivalents (CO2e) (kg energy-corrected milk (ECM))-1, encompasses emissions from feed production, diet management and total product output. The proportion of pasture on diets may affect all these factors, mainly in subtropical climate zones, where cows may access tropical and temperate pastures during warm and cold seasons, respectively. The aim of the study was to assess the C footprint of a dairy system with annual tropical and temperate pastures in a subtropical region. The system boundary included all processes up to the animal farm gate. Feed requirement during the entire life of each cow was based on data recorded from Holstein × Jersey cow herds producing an average of 7,000 kg ECM lactation-1. The milk production response as consequence of feed strategies (scenarios) was based on results from two experiments (warm and cold seasons) using lactating cows from the same herd. Three scenarios were evaluated: total mixed ration (TMR) ad libitum intake, 75, and 50% of ad libitum TMR intake with access to grazing either a tropical or temperate pasture during lactation periods. Considering IPCC and international literature values to estimate emissions from urine/dung, feed production and electricity, the C footprint was similar between scenarios, averaging 1.06 kg CO2e (kg ECM)-1. Considering factors from studies conducted in subtropical conditions and actual inputs for on-farm feed production, the C footprint decreased 0.04 kg CO2e (kg ECM)-1 in scenarios including pastures compared to ad libitum TMR. Regardless of factors considered, emissions from feed production decreased as the proportion of pasture went up. In conclusion, decreasing TMR intake and including pastures in dairy cow diets in subtropical conditions have the potential to maintain or reduce the C footprint to a small extent.

Fig 1. Overview of the milk production system boundary considered in the study.

Fig 2. Overall greenhouse gas emissions in dairy cattle systems under various scenarios.

TMR = ad libitum TMR intake, 75TMR = 75% of ad libitum TMR intake with access to pasture, 50TMR = 50% of ad libitum TMR intake with access to pasture. (a) N₂O emission factors for urine and dung from IPCC [38], feed production emission factors from Table 3 without accounting for sequestered CO₂-C from perennial pasture, production of electricity = 0.73 kg CO₂e kWh^-1 [41]. (b) N₂O emission factors for urine and dung from IPCC [38], feed production emission factors from Table 3 without accounting for sequestered CO₂-C from perennial pasture, production of electricity = 0.205 kg CO₂e kWh^-1 [46]; (c) N₂O emission factors for urine and dung from local data [37], feed production EF from Table 4 without accounting for sequestered CO₂-C from perennial pasture, production of electricity = 0.205 kg CO₂e kWh^-1 [46]. (d) N₂O emission factors for urine and dung from local data [37], feed production emission factors from Table 4 accounting for sequestered CO₂-C from perennial pasture, production of electricity = 0.205 kg CO₂e kWh^-1 [46].

Fig 3. Sensitivity of the C footprint.

Sensitivity index = percentage change in C footprint for a 10% change in the given emission source divided by 10% of. (a) N₂O emission factors for urine and dung from IPCC [38], feed production emission factors from Table 3, production of electricity = 0.73 kg CO₂e kWh^-1 [41]. (b) N₂O emission factors for urine and dung from IPCC [38], feed production emission factors from Table 3, production of electricity = 0.205 kg CO₂e kWh^-1 [46]; (c) N₂O emission factors for urine and dung from local data [37], feed production EF from Table 4 without accounting sequestered CO₂-C from perennial pasture, production of electricity = 0.205 kg CO₂e kWh^-1 [46]. (d) N₂O emission factors for urine and dung from local data [37], feed production emission factors from Table 4 accounting sequestered CO₂-C from perennial pasture, production of electricity = 0.205 kg CO₂e kWh^-1 [46].

Fig 4. Greenhouse gas emissions (GHG) from manure and feed production in dairy cattle systems.

TMR = ad libitum TMR intake, 75TMR = 75% of ad libitum TMR intake with access to pasture, 50TMR = 50% of ad libitum TMR intake with access to pasture. (a) N₂O emission factors for urine and dung from IPCC [38]. (b) Feed production emission factors from Table 3. (c) N₂O emission factors for urine and dung from local data [37]. (d) Feed production emission factors from Table 4 accounting sequestered CO₂-C from perennial pasture.