A dairy farm greenhouse gas (GHG) model was applied in this study to compare the Intergovernmental Panel on Climate Change (IPCC) method and the life cycle assessment (LCA) procedure, which are the principal methods for quantifying the carbon footprint of dairy production. The objectives of this paper were to compare the auditing methods in estimating the carbon footprint of grass and confinement dairy systems and to assess the methods in estimating the footprint of grass-based dairy farms varying in cow genetic potential, stocking rate and level of concentrate feeding. The input data used to operate the model was based on published research studies. The results of the study showed that the IPCC and LCA methods ranked the carbon footprint of dairy systems differently. For example, the IPCC method found that the carbon footprint of the confinement dairy system was 8% lower than the grass system, but the LCA results show that the confinement system increased the carbon footprint by 16%. The comparison of grass-based dairy systems, differing in cow genotype, stocking rate and concentrate fed per cow also showed that the methods did not agree on the ranking of dairy systems carbon footprint. The re-ranking of dairy systems carbon footprint occurred because the IPCC method excludes emissions associated with imported goods, for example, concentrate. Thus, it is incorrect to consider only components of the dairy system relevant for policy reporting such as that used by IPCC when estimating the carbon footprint of dairy produce. Instead, holistic approaches, such as LCA, which consider on and off-farm GHG emissions should be used. Therefore, reform of the present policy framework is required to enable quantification of the impact of mitigation strategies on global emissions. The evaluation of the carbon footprint from grass-based systems differing in cow genotype also demonstrated that selecting cows solely for milk production will increase the carbon footprint of grass-based dairy systems relative to cows selected on a combination of traits, because of reduced cow fertility and thus higher emissions from replacement heifers.

Nitrogen (N) losses from dairy production systems are a cause for environmental concern. Excreted primarily as urea N in the urine, this volatile form of N can be lost as ammonia (NH3) contributing to ground-level ozone, the greenhouse effect and the deterioration of terrestrial and aquatic ecosystems. In addition, the production of urea N places a metabolic demand for energy on the dairy cow and excessively high levels of blood urea N are known to have deleterious effects on reproductive performance. Therefore, it is of interest to develop strategies that reduce N excretion from dairy cows and to this end, dietary manipulation of N efficiency offers great potential. There are a significant number of reports in the literature on N efficiency in the lactating dairy cow, including reducing dietary CP intake, improving the balance of amino acids reaching the small intestine, optimising the forage mix and optimising the energy sources in the diet. Across these experiments, N intake ranged from 0.33 to 0.67 kg/day with N efficiency ranging from 0.21 to 0.42. This paper will report on recent N balance experiments conducted at University College Dublin, as well as reports in the literature on studies aimed at improving N efficiency in the lactating dairy cow.

Nitrogen (N) losses and ammonia emissions from livestock operations represent a significant water and air pollutant. It has been repeatedly demonstrated that reduction in dietary protein input results in improved N utilization efficiency and reduced N losses with manure in dairy cows. Reducing dietary protein can also benefit the producer by reducing feed cost. These interventions, however, have to be balanced with the risk of decreased productivity. If animal requirements for metabolizable protein (MP) are not met, productivity cannot be sustained. Supplementation with rumen-protected (RP) amino acids (AA) limiting milk production and milk protein synthesis may compensate for the lack of MP in dairy cow diets. Long-term experiments at Penn State University demonstrated reduced urinary N losses, blood urea, milk urea N, and ammonia emissions from manure with MP-deficient v. MP-adequate diets. In some trials, however, milk production and milk protein yield were decreased with the MP-deficient diets. Supplementation of these diets with RPAA (Lys, Met and His) increased dry matter intake and milk yield of the cows to levels comparable with MP-adequate diets and can be a successful strategy for reducing N losses and maintaining productivity in lactating dairy cows fed low-protein diets.

Next to dry matter (DM) intake, nutritional factors cause considerable variation in methane (CH4) emitted and nitrogen (N) excreted per kg of DM intake or per kg of milk. Rumen function in particular determines CH4 emission and concomitant (amount and site) of N excretion, including the trade-offs between them with changes in nutrition and cow characteristics. Quantification of the interaction between CH4 and N emission hence requires quantification of effects on rumen function in particular. The models available to quantify CH4 emission require the same types of input. The detail of questions posed determines the choice of model and the required level of detail of model inputs needed to investigate mitigation measures and the interaction between CH4 and N emission for a specific farming case. Simulation results with a mechanistic model of enteric fermentation confirmed a profound impact of nutritional measures on both CH4 and N emission, but also demonstrated that nutritional measures to mitigate N excretion can be associated with an increase in CH4 emission. This result demonstrates the need to consider details on the rumen level when the aim is to quantify accurately the net effect on greenhouse gas emission for a specific case studied, which contrasts with applying generic values. As an alternative to models of quantification, on-farm measurement of emission might be pursued by sampling of excreta and air. The principle problem is that concentrations are measured which not necessarily reflect daily rates. Milk production rate is recorded on-farm however, which makes indicators based on milk composition just as promising candidates to estimate CH4 (milk fat) or N (milk urea) emission, provided bias by variation in milk composition unrelated to CH4 and N emission rate can be prevented.

Animal breeding should be considered as a permanent and cumulative approach to reducing the environmental footprint of dairy cattle production systems within an overall national and global mitigation strategy. Current international dairy cattle breeding goals do not explicitly include environmental traits, but observed improvements in milk production and both fertility and longevity contribute substantially to improving the environmental footprint relative to output. Ideally, however, environmental related traits, most notably greenhouse gas emissions and nitrogen excretion, should be explicitly included in national breeding goals with their own economic weight. Access to routine phenotypic observations for the environmental traits or other information including genomic information or information on heritable correlated traits is required for inclusion in the selection index. There is, however, a considerable paucity of information on the genetic parameters for, in particular, greenhouse gas emissions in dairy cattle; these parameters include genetic variance estimates, as well as genetic and phenotypic (co)variances with other performance traits. Large studies with well phenotyped animals across a range of environments are needed to estimate such parameters and also investigate the extent, if any, of genotype-by-environment interactions across contrasting environments. Considerable genetic variation in milk urea nitrogen, as a proxy for nitrogen excretion in the urine, exist and suggest that breeding programmes to improve nitrogen use efficiency will be fruitful. However, because of the antagonistic genetic correlations between milk urea nitrogen and milk production, genetic gain in milk yield is expected to be compromised within a breeding goal that includes milk urea nitrogen.

Phosphorus (P) is an essential mineral that needs to be supplied in sufficient quantities for maintenance and growth and milk production in dairy cattle. However, over 60% of the P consumed can be excreted in faeces with a potential to cause environmental pollution. Concern over higher levels of P in intensively managed livestock systems has led to legislation such as the Water Framework Directive in the European Union. In this manuscript, several methods of reducing P pollution are discussed. A major source of environmental P pollution has been overfeeding P mainly due to addition of ‘safety margin’ over the animal's requirement and concerns related to fertility. Matching the animal's requirement and feeding in groups so that animals at the same physiological status are fed according to their requirement has a potential to reduce P excretion significantly. P can also be reduced by matching available P with the metabolizable energy content of the diet because more P can be incorporated into milk when P is utilized by rumen microbes, which are limited by energy. Plants contain phytate bound P that need to be broken up before they can be absorbed by the animal. Although ruminants can digest phytate, use of phytase enzyme could help either directly by acting on phytate P or improvement of feed digestibility. Pasture management can lead to improved nutrient cycling, particularly if the soil is deficient in P. However, overfertilizing pasture could result is higher runoff of dissolved reactive P. Management practices that leave adequate forage residue on the surface such as rotational grazing will improve infiltration and decrease runoff, reducing nutrient losses.

Nitrous oxide reductase (N2OR), the enzyme responsible for the reduction of N2O to N2 in denitrification, uses copper (Cu) as its cofactor. Its activity is lowered under conditions of Cu deficiency. In general, high organic matter (OM) soil decreases Cu availability. The present study investigated different Cu forms, namely organic (ORG) v. inorganic (INO), and associated concentrations (750, 550, 125, 60 μg Cu/g soil) for their efficacy in affecting denitrification and especially N2OR activity in high OM peat soil in a water saturated anaerobic condition for 24 h. Gas and liquid samples were taken every 8 h and analyzed for NO3−, NO2−, N2O and N2. Inorganic Cu treatments did not affect N transformation rates and N2OR activity among the different treatments (P > 0.05) throughout the incubation compared with the control (CON). The ORG Cu treatments increased NO3− (P < 0.05), NO2− (P < 0.05) and N2O (P < 0.05) transformation rates compared with CON. These changes were ORG Cu dose dependent. N2OR activity increased first in the 750 μg ORG Cu treatment (P < 0.05) during 8 to 16 h followed by the other ORG Cu treatments (P < 0.05) during 16 to 24 h compared with CON. These results highlight the importance of Cu form and concentration on N transformation rate during denitrification. The findings can potentially be applied to systems like soil, wastewater, constructed wetlands, etc., in which reactions of the denitrification pathway are manipulated.