Environmental Impact of Rotationally Grazed Pastures at Different Management Intensities in South Africa

Abstract

Nitrogen fertilization, irrigation and concentrate feeding are important factors in rotational pasture management for dairy farms in South Africa. The extent to which these factors affect environmental efficiency is subject to current and intense debate among scientists. A three-year field study was conducted to investigate the yield response of different N-fertilizer treatments (0 (N0), 220 (N20), 440 (N40), 660 (N60) and 880 (N80) kg N ha−1 year−1) on grazed pastures and to calculate the carbon footprint (CF) of milk produced. Excessive N-fertilization (N60 and N80) did not increase herbage dry matter and energy yields from pastures. However, N80 indicated the highest N-yield but at the same time also the highest N surpluses at field level. A maximum fertilizer rate of 220 kg ha−1 year−1 (in addition to excreted N from grazing animals) appears sufficient to ensure adequate herbage yields (~20 t DM ha−1 year−1) with a slightly positive field-N-balance. This amount will prevent the depletion of soil C and N, with low N losses to the environment, where adequate milk yields of ~17 t ECM ha−1 with a low CF (~1.3 kg CO2 kg ECM−1) are reached. Methane from enteric fermentation (~49% ± 3.3) and N2O (~16% ± 3.2) emissions from irrigated pastures were the main contributors to the CF. A further CF reduction can be achieved by improved N-fertilization planning, low emission irrigation techniques and strategies to limit N2O emissions from pasture soils in South Africa.

A carbon footprint assessment for pasture-based dairy farming systems in South Africa

“…The average carbon footprint across all farming systems was 1.36 ± 0.21 kg CO 2 eq kg −1 FPCM, which was in alignment with (Meissner and Ohlhoff, 2022) who reported GHG emissions from milk production in South Africa range between 1.2 and 1.4 kg CO 2 eq kg −1 milk. Smit et al (2021) measured GHG emissions directly from soil in response to N fertilizer treatments in the Western Cape province of South Africa and reported a carbon footprint of 1.3 kg CO 2 eq kg −1 energy corrected milk, for treatments receiving <200 kg N ha −1 year −1 , and a maximum carbon footprint of 2.6 kg CO 2 eq kg −1 ECM when 800 kg N ha −1 year −1 was applied. Compared to similar cradle-to-farmgate lifecycle assessment of GHG emissions studies performed outside South Africa, the average carbon footprint found in our assessment was greater.…”

“…Despite the growing interest in dairy-based carbon footprint studies globally, assessments for South African pasture-based dairy farming systems are currently absent (Smit et al, 2021;Mazzetto et al, 2022) although livestock accounts for ∼50% of annual CH 4 emissions in South Africa (Department of Forestry, Fisheries and the Environment, 2021). In addition, dairy cattle account for ∼12% of the annually emitted enteric CH 4 from all livestock in South Africa (Tongwane and Moeletsi, 2020).…”

Excessive Nitrogen Fertilization Is a Limitation to Herbage Yield and Nitrogen Use Efficiency of Dairy Pastures in South Africa

“…Considering the many avenues of nutrient inputs in dairy pasture systems, overfertilization is likely [8,9]. Pasture productivity deteriorates when fertilizer is applied in excess [10].…”

“…Furthermore, over-fertilization results in losses through leaching and run-off of nutrients, which impairs water quality at the ground and surface water levels [11,12]. Overfertilizing with N also increases greenhouse gas emissions, particularly nitrous oxide [9]. It is thus important that producers refine fertilizer programs to match crop requirements and consider the amount of soil N that could be mineralized from organic matter [8].…”