Improving production efficiency as a strategy to mitigate greenhouse gas emissions on pastoral dairy farms in New Zealand

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Abstract

New Zealand's commitment to the Kyoto Protocol requires agriculture, including dairy farming, to reduce current greenhouse gas (GHG) emissions by about 20% by 2012. A modeling exercise to explore the cumulative impact of dairy management decisions on GHG emissions and profitability is reported. The objective was to maintain production, but reduce GHG emissions per unit of land and product by improving production efficiency. A farm-scale computer model that includes a mechanistic cow model was used to model an average, pasture-based New Zealand farm over different climate years. A mitigation strategy based on reduced replacement rates was first added to this baseline farm and modeled over the same years. Three more strategies were added, improved cow efficiency (higher genetic merit), improved pasture management (better pasture quality), and home-grown maize silage [increased total metabolizable energy (ME) yield and reduced nitrogen intake], and modeled to predict milk production, intakes, methane, urinary-nitrogen, and operational profit. Profit was calculated from 2006/2007 economic data, where milksolids (fat + protein) payout was NZ$ 4.09 kg−1.1 A nutrient budget model was used with these scenarios and two more strategies added: cows standing on a loafing pad during wet conditions and application of a nitrification inhibitor to pasture (DCD). The nutrient budget model predicted total GHG emissions in CO2 equivalents and included some life cycle analysis of emissions from fertilizer manufacturing, fuel and electricity generation. The simulations suggest that implementation of a combination of these strategies could decrease GHG emissions by 27–32% while showing potential to increase profitability on a pasture-based New Zealand dairy farm. Increasing the efficiency of milk production from forage may be achieved by a combination of high (but realistic) reproductive performance leading to low involuntary culling, using crossbred cows with high genetic merit producing 430 kg milksolids yr−1, and pasture management to increase average pasture and silage quality by 1 MJ ME kg dry matter−1. These efficiency gains could enable stocking rate to be reduced from 3 to 2.3 cows ha−1. Nitrogen from fertilizers would be reduced to less than 50 kg ha−1 yr−1 and include “best practice” application of nitrification inhibitors. Considerable GHG mitigation may be achieved by applying optimal animal management to maximize efficiency, minimize wastage and target N fertilizer use.

Introduction

In New Zealand, methane (CH4) contributes 38% and nitrous oxide (N2O) 17% (CO2 equivalents; CO2-e) of the annual emissions (NZ Climate Change Office, 2003). Agriculture contributes about half of New Zealand GHG emissions, most of them coming from grazed pasture-based livestock production systems. In these systems, enteric fermentation and urinary-nitrogen (urinary-N) are the most important sources of CH4 and N2O (Waghorn, 2008). Previous studies have summarized the current and future strategies available to pasture-based farmers for reducing GHG emissions by animal, feed-based, soil and management interventions (Beauchemin et al., 2008, de Klein and Eckard, 2008). There is a need to evaluate the impacts of these strategies when incorporated into the farm system and also the cumulative effects when some of these strategies are combined. Furthermore, variability, as influenced by climate and animal-feed dynamics, needs to be considered (Beauchemin et al., 2008). Farm-scale models are cost effective ways of exploring the cost/benefits of practical and multiple mitigation options over several years.

Dairy farming in New Zealand is responsible for about 36% of agricultural GHG emissions (Ministry for the Environment, 2008). Seasonal calving dairy cows are fed ryegrass-dominant pastures. Typically, all cows calve at the end of winter (July–September) and are milked for 8–9 months so feed requirements are largely met through fresh pasture. Supplements are typically up to 10% of feed intake, sometimes bought from outside the farm (and overseas), or else grown on farm. Grains are rarely fed but silages are important.

The objective of this modeling exercise was to evaluate the cumulative efficacy of selected mitigation strategies and to calculate their effects on farm profitability. The hypothesis was that improved farm efficiencies may be used to mitigate GHG emissions and increase profitability without affecting production. The rationale was that feed intake is the main driver of GHG emissions on the dairy farm, and improved efficiency would reduce total feed use (i.e. of the whole herd including replacements) for the same level of milk production. The following strategies were included:

  • Reduction in the numbers of replacement and other non-productive animals. Non-productive animals produce CH4 and urinary-N without contributing to milk production (Waghorn, 2008).

  • Increasing the feed conversion efficiency using animals with higher genetic merit. Efficient cows produce more milk from the same energy intake and CH4 output. Fewer efficient animals are required to produce the same milksolids (MS; protein + fat) per unit of land, and because less feed is required so less CH4 should be emitted and less urinary-N is deposited (de Klein and Eckard, 2008).

  • Increasing pasture quality to achieve a higher average metabolizable energy (ME) content (Beauchemin et al., 2008). With high ME pasture, less feed is required to produce the same output per unit of land, resulting in lower CH4 emissions and less urinary-N deposited. Furthermore, because less feed is required (of better quality) less N-fertilizer is required, resulting in savings in GHG generated during the fertilizer manufacturing process (Wells, 2001).

  • Growing a maize crop on part of the farm will increase ME yield per hectare because the yield from maize is higher than from pasture, and a lower pasture yield from the rest of the farm will be required to produce the required ME, hence less N-fertilizer is required for pasture, with reduced N2O loss from fertilizer as well as CO2-e from the fertilizer manufacture. Feeding maize silage to cows will also lower urinary-N excretion and, therefore, N2O loss from urine patches (Van Vuuren et al., 1993).

  • Application of nitrification inhibitors (e.g. DCD) in autumn and winter to slow the process of nitrification and reduce the losses of N2O. More N remains in the soil for pasture growth allowing lower fertilizer rates (de Klein and Eckard, 2008).

  • Standing cows on loafing pads to capture excreta and also reduce pasture damage during wet conditions (standing off). Captured excreta can be re-cycled to pastures for efficient utilization of N by plants (de Klein and Eckard, 2008) and the reduction in N-fertilizer use lowers GHG emissions associated with its manufacture. By reducing pugging and soil compaction, N2O emissions from soils can also be reduced.

Section snippets

Approach

Information from DairyBase (www.dairybase.co.nz) was used to describe a pasture-based, self-contained (<10% bought-in feed), ‘average’ dairy farm in the Waikato region of New Zealand. This baseline farm did not implement specific strategies to reduce GHG emissions. Mitigation strategies were then sequentially added to this baseline farm, based on performance indicators from top-performing farms, and modeled through DairyNZ's Whole Farm Model (WFM; Beukes et al., 2008) with the Molly cow model (

Results

When the farm scenarios were developed, the intention was to maintain milk production constant (within acceptable limits) across the five farms. However, since milk production is an output, in the simulations of the five scenarios over consecutive years, it was influenced by herd reproductive performance of the previous year, dry-off decisions, and climate-driven feed dynamics. This resulted in the average milk production for Farm A being slightly lower compared to the four other farms (Table 2

Discussion

Both production (products per hectare) and GHG emissions from grazed pastoral systems are mainly driven by stocking rate and, therefore, total DM intake (de Klein et al., 2008). An increase in production efficiency can either result in more production for the same DM intake, but not necessarily with any net reduction in emissions, or less intake (and emissions) for the same production. These differences are important when evaluating management strategies that can have the largest reduction in

Conclusions

If the assumptions used in the simulations could be implemented on a Waikato dairy farm in New Zealand there is potential to decrease GHG emissions by 27–32% while there is an opportunity to increase profitability by saving on cow and fertilizer costs. The key lies in maintaining production and lowering total DM intake. This can be achieved by a combination of high (but realistic) reproductive performance leading to lower involuntary culling, use of crossbred cows with high genetic merit and

Acknowledgements

This work was funded by the Pastoral Greenhouse Gas Research Consortium. Valuable contributions to this manuscript were made by Dave Clark, Cameron Clark, Chris Glassey, Eric Hillerton and Bruce Thorrold.

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