Benefits Are Limited with High Nitrogen Fertiliser Rates in Kikuyu-Ryegrass Pasture Systems

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

The response of crop yields to fertilizers is a long-standing topic of agricultural production. Currently, in dairy-pasture systems, nitrogen (N) fertilizer is used as a management tool that is said to be directly proportional to pasture yield. We evaluated a large dataset consisting of data from 153 fields over five years to examine the effects of N fertilization on pasture yield and nitrogen use efficiency in the Eastern Cape province of South Africa. Fertilizer application rates were grouped into three treatments viz., <200, 200–350, and >350 kg N ha−1, and herbage yield response over the years was analyzed with mixed models. There were no differences found between treatments for total annual herbage yield over the years. High N fertilizer rates did not translate to a higher herbage yield of pastures. The N rate had a weak but significant negative correlation with the total annual yield and only accounted for 6% of the yield variation. The N use efficiency of pastures improved with reduced N application rates. Pasture yield varies through different seasons. Spring and summer account for the highest yield, coinciding with warm and moist conditions favorable for N mineralization in the soil. Farmers need to consider the time of the year and plan their monthly or seasonal fertilizer application accordingly to account for peak N mineralization rates.

In vitro Fermentation Profile and Methane Production of Kikuyu Grass Harvested at Different Sward Heights

Highly digestible forages are associated with an in vitro low-methane (CH4) rumen fermentation profile and thus the possibility of reducing CH4 emissions from forage-based systems. We aimed to assess the in vitro ruminal fermentation profile, including CH4 production, of the top stratum of Kikuyu grass (Cenchrus clandestinus - Hochst. ex Chiov) harvested at different sward heights (10, 15, 20, 25, and 30 cm). Herbage samples (incubating substrate) were analyzed for their chemical composition, in vitro organic matter digestibility (IVOMD), and morphological components. In vitro incubations were performed under a randomized complete block design with four independent runs of each treatment. Gas production (GP), in vitro dry matter digestibility (IVDMD), CH4 production, total volatile fatty acid (VFA) concentration, and their acetate, propionate, and butyrate proportions were measured following 24 and 48 h of incubation. Herbage samples had similar contents of organic matter, neutral detergent fiber, and crude protein for all treatments. However, a higher acid detergent fiber (ADF) content in taller sward heights than in smaller sward heights and a tendency for metabolizable energy (ME) and IVOMD to decrease as sward height increased were found. Similarly, the stem + sheath mass tended to increase with increasing sward height. Amongst the nutrients, ME (r = −0.65) and IVDMD (r = −0.64) were negatively correlated with sward height (p &lt; 0.001) and ADF was positively correlated with sward height (r = 0.73, p &lt; 0.001). Both the GP and IVDMD were negatively related to the sward height at both incubation times. Sward heights of Kikuyu grass below 30 cm display an in vitro profile of VFAs high in propionate and low in acetate, with a trend toward lower methane production of CH4 per unit of IVDMD. These findings are important to aid decision-making on the optimal sward height of Kikuyu grass and manage animal grazing with the opportunity to reduce CH4 production.

Toward Specialized or Integrated Systems in Northwest Europe: On-Farm Eco-Efficiency of Dairy Farming in Germany

Intensive confinement (IC) systems for dairying have become widespread during the last decades. However, potential advantages of alternative systems such as full-grazing (FG) or integrated dairy/cash-crop (IFG) systems with regards to better provision of ecosystem services are widely discussed. To investigate performance and environmental impacts, we compared four prevailing dairy systems using an on-farm research study. The farm types differed in their share of pasture access and quantity of resource inputs: (i) an IC with a high import of supplements and mineral fertilizers; (ii) a semi-confinement (SC) with daytime pasture access during summer and moderate import of supplementary feeds representing the base-line scenario; (iii) a FG based on grazed seeded grass-clover swards with no purchased N-fertilizers and low quantities of supplementary feeds; and (iv) an IFG comparable to FG but based on grass-clover leys integrated in a cash-crop rotation. Results revealed highest milk productivity (16 t energy-corrected-milk (ECM) ha−1) and farm-N-balance (230 kg N ha−1) in IC; however, the highest product carbon footprint (PCF; 1.2 CO2eq kg ECM−1) and highest N-footprint (13 g N kg ECM−1) were found in the baseline system SC. The FG and IFG revealed on average similar forage dry matter yields (10 – 11 t DM ha−1) at similar crude protein and net-energy-lactation ratios per kg DM-intake compared to the IC and SC. The PCF in FG were comparable to IC (0.9 vs. 1.1 kg CO2eq kg ECM−1) but at a lower N-footprint (9 vs. 12 g N kg ECM−1). However, despite low measured N-losses in the FG system, the farm-N-surplus was exceeded by 90 kg N ha−1. A further reduction was only possible in the IFG (50 kg N ha−1) by accounting for a potential N-carry-over from N-rich plant residues to the cash-crop unit, leading to the lowest PCF (0.6 kg CO2eq kg ECM−1) for the IFG, with still moderate milk yield levels (~10,500 kg ECM ha−1). According to this bottom-up approach based on field data, improved integrated grazing systems could provide an important opportunity to increase the ecosystem services from dairy farming, operating with land use efficiencies similar to IC.

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

Simple Summary

Nitrogen fertilization, irrigation and concentrate feeding are key management factors in grazed dairy-pasture systems. However, the extent to which these management factors affect environmental efficiency is a current debate among scientists. We designed a study to investigate dairy system environmental efficiency through the evaluation of the carbon footprint of milk and the nitrogen-balance as a result of different fertilization rates applied on irrigated dairy-pastures in South Africa. The lowest carbon footprint was observed when low rates of fertilizer were applied. Methane from ruminal digestion and nitrous oxide emissions from grazed pastures were the main contributors affecting the carbon footprint. The application of fertilizer resulted in only small herbage yield differences between treatments. The nitrogen-balance was negative when no nitrogen was applied. This indicates that such an approach will not be sustainable without adapting sward species composition (e.g., introduction of forage legumes), even though nitrogen circulates through animal manure to the pasture. The environmental impact of milk produced from pasture-based dairy farms can be reduced through increased farm nitrogen use-efficiency and improved irrigation systems 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.