Breeding a sustainable future for milk production

Selective breeding to improve the productivity of livestock systems has been carried out for centuries as a cost-effective approach to bring about rapid and permanent change¹. Genetic improvement in animals and plants can not only enhance food production² but also address the challenge of mitigating greenhouse gas emissions from food³. Improving the genetic potential of animals for production, their reproductive efficiency and lifespan, health, and lifetime productivity are highly effective approaches for enhancing animal production efficiency and thus reducing emissions per unit of product⁴. Genetic selection can partly help to address some of these performance challenges. In the 20th century, livestock breeding was focused on productivity in many countries around the world, such as milk and meat yields and their composition, until it became apparent that selecting livestock solely on production traits had a detrimental effect on health, fertility and lifespan⁵. Therefore, over the last 20 years, the need to select animals on a balance of more heritable production traits (e.g., milk and meat yield) and less heritable fitness traits (e.g., health, fertility and lifespan) has gained importance in many countries given economic, environmental and social imperatives^6,7. The health and welfare of livestock are of great importance to society, and ultimately poor health and fertility lead to inefficiencies and wasted resources that are then linked to less sustainable systems⁸. The extra feed intake required per unit product is not only costly and often between 50% and 70% of production costs, but also dietary nutrients are lost in the form of enteric and manure greenhouse gas emissions (e.g. methane and nitrous oxide). In dairy cows, the genetic selection on health, fertility and lifespan has been found to reduce the carbon footprint of milk largely due to a dilution in maintenance requirements as fewer resources and animals are required to produce a given amount of milk^9,10.

Various national economic and environmental breeding indices have been developed based on an increasing range of phenotypic traits being recorded to meet the expectations of modern society (such as the Profitable Lifetime, Envirocow, and Healthy Cow indices for dairy cows in the UK), and production often has a similar magnitude of weighting to fitness traits. However, research¹¹ has found that for an index focused on carbon, rather than profit, to select cows on their genetic potential to reduce herd emissions the weight placed on fitness traits would potentially be higher. Therefore, a higher weighting on fitness traits in a selection index should put more emphasis on environmental and social priorities. While genetic selection in dairy cows has brought about a remarkable reduction in the greenhouse gas emissions per unit of milk over several decades^12,13,14, the selection of traits associated with carbon emissions should help bring about a permanent and cumulative reduction in emissions per cow as well as per unit product.

This paper compared the performance of a traditional economic index approach to a carbon index based on detailed historical lifetime records for dairy cows to assess the balance of traits for more sustainable milk production in national populations. Economic and carbon coefficients derived in a previous study¹¹ were applied to 1042 female dairy cows with genetically predicted transmitting abilities (PTAs) for a range of biological traits. The PTA represents the genetic potential of a cow and is the estimated increase or reduction in a single trait that a cow will transmit to their offspring relative to a cow with an average PTA of zero. Based on the historical lifetime records obtained, this paper demonstrates that selective breeding in dairy cows should place a greater emphasis on non-milk production traits (i.e. fertility, lifespan and efficiency traits) to enhance resource use and reduce the absolute carbon footprint of milk production. Cows with a positive economic index and a negative carbon index are more sustainable animals.

There was a high negative rank correlation between economic and carbon index values among cows (rho = −0.72; Fig. 1). The rank correlation between economic and carbon index values for cows was fairly consistent across lifetime parities with −0.72 for first parity, −0.80 for second parity, −0.66 for third parity, −0.70 for four parity and −0.60 for fifth or greater parity. The average (±s.d.) cow during the 10-year period was estimated to reduce herd profit by −£27 (±105) per cow per lactation with a range of −£381 to £253 per cow and reduce the carbon footprint of the herd by −20 (±293) kg CO₂-eq. per cow per lactation with a range of −784 to 790 kg CO₂-eq. per cow. A 10% increase in economic and carbon coefficients for fitness traits (somatic cell count, lifespan and calving interval) would result in a greater range of economic index values from −£409 to £270 per cow and carbon index values from −838 to 864 kg CO₂-eq. per cow per lactation. Overall, 31% of cows were considered desirable with both a positive economic and a negative carbon value, and would be classed as sustainable; 41% of cows had a positive economic value and 45% a negative carbon value. Sustainable cows had a lower average PTA for milk volume, milk fat and protein, somatic cell count, calving interval, and a higher average PTA for lifespan than cows classed as not sustainable (Table 1).

Economic and carbon coefficients were applied to individual animal PTA values for key biological traits to derive the overall economic and carbon index value for each animal.

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As expected, lifespan in breeding livestock is largely related to the health and fertility performance of the animal, and this study showed that lifespan was highly correlated with cows that ranked highly on both an economic and carbon index (rh = 0.91 and −0.91, respectively; Table 2). Fertility also had a high-rank correlation with carbon index values (rho = 0.76).

In the herd studied, the percentage of sustainable breeding cows born increased considerably after the year 2018, which reflected an increased emphasis on breeding animals with a higher PTA for fertility and lifespan (Table 1). The percentage of animals classified as sustainable ranged from 9% to 98% during the period studied (Fig. 2).

A sustainable animal has a positive economic value and a negative carbon index value.

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The objective of breeding has historically been towards increasing the profitability of production, and hence the popularity of the use of economic selection indexes around the world for several decades. However, this tends to put more emphasis on traits associated directly with output, such as milk production traits, rather than traits indirectly associated with production, such as health and fertility. The consequences of this are we have seen a continued reduction in the emissions intensity per unit milk production of between 0.6% and 1% per year in countries that have implemented a national economic breeding index^12,13,14. However, if the breeding objective aimed to profitably reduce your carbon footprint per unit product and per animal, and ultimately be more sustainable, then a balanced weighting on milk production, fitness and efficiency traits in multi-trait selection is recommended and reductions in emissions per cow of 0.5–1% per year may be possible¹⁵, i.e. a combination of economic and carbon indexes. A carbon index per cow places more weighting on non-milk production traits, as shown in Table 3, and notably less weighting on milk protein which is a valuable trait to producers. In national breeding indices there has been a noticeable reduction in emphasis placed on milk production traits towards those related to efficient resource use including health, fertility, survival and feed intake¹⁶. The Nordic Total Merit index is currently the only multi-trait selection index for dairy cattle to maintain a balanced weighting across production, fitness and functional traits over the years and other countries are adopting a similar approach^16,17. It is recognised that lifespan in breeding animals is measurable and critical to lifetime productivity, along with fertility⁷. A high rank correlation for lifespan was associated with more sustainable cows (i.e. cows with a positive economic and negative carbon index value). However, the weighting that different countries place on lifespan and fertility can range from zero to about 25% compared to other traits in national multi-trait breeding indices^6,17. The herd studied had a higher economic and carbon weighting for lifespan compared to the national population of cows (Table 3).

It is well known that the selection of dairy cows on milk production traits has a deleterious effect on health and fertility, and overall lifespan⁵. Improved fertility has been shown to reduce herd emissions through less replacement animals and resources needed to maintain the same size herd¹⁸. Unless more emphasis is placed on fertility and reproductive performance, the need for more replacement animals and the impact on carbon footprint will still remain an issue. Choosing suitable parents to breed the next generation of milking cows is a critical decision for any dairy farmer, and it involves several factors to consider. With modern advanced recording systems for animal performance data there are an increasing number of direct phenotypic and indirect genomic derived traits available e.g. milk, health, fertility, survival, with differing heritability and correlations between traits. The current study included common traits that are directly measured on farms, and have been found to be associated with greenhouse gas emissions from milk production^13,15. Within the traits studied, there was considerable genetic variability observed among the cows, with a large range in PTAs (Table 1). This variability in PTA values among cows contributed to the range of cows classed as being sustainable, which has increased in recent years and reflected an increased emphasis on breeding animals for improved fertility and lifespan.

Furthermore, with the environmental and societal importance placed on resource use and pollution, new genomic PTA values for feed intake and enteric methane emissions are becoming available and are important for sustainability^8,19,20. The cows in the current study were not genotyped but doing so to derive genomic breeding values for efficiency traits such as enteric methane emissions and feed intake would be beneficial for reducing the carbon footprint of dairy cows. Including enteric methane in a multi-trait index would be more important in a carbon index than an economic index given its predicted weighting (23% compared to 1%, respectively, for the UK national population; Table 3), and therefore reducing emissions with little effect on profitability. Whereas, including feed intake in multi-trait selection to select more feed efficient cows per unit milk would have a relatively high weighting and importance in both a carbon and economic index (27% compared to 10% respectively for the UK national population). Including both feed intake and enteric methane traits in an economic multi-trait index will result in a slight reduction in weighting for both production and fitness traits. In a carbon index per cow feed intake and enteric methane would have a similarly high weighting along with lifespan. The ultimate aim of any modern dairy breeding programme has to be towards a desired goal of more sustainable production based on a range of desirable traits for improvement, e.g. milk production, health, fertility and feed resource use. As the study shows, both productivity gains and absolute carbon reductions are possible. Food production must recognise the importance of ‘futurity’²¹ in sustainability. Developments in milk production have met the needs of the present through genetics, nutrition and management, but must adapt to meet the human and animals’ needs of future generations by addressing the balance of traits to reduce livestock emissions.

The carbon index per cow approach (Eq. (2)) in the current study could be applied to national genetic reporting and PTA values to provide individual farms with index values for their female and male breeding animals to identify those that are more sustainable, with a higher weighting on fitness, feed and methane traits. Furthermore, the CO₂-equivalent values per cow per lactation could provide opportunities for farms to claim carbon credits (i.e. credits per tonne of CO₂-equivalent) where permanent and cumulative reductions are demonstrated over the lifetime of an animal. Various countries in the world are now adopting both economic and environmental breeding indices^19,20 with more equal weighting placed on milk production, survival and feed efficiency traits e.g. Profitable Lifetime Index and Envirocow in UK and Balanced Performance Index and Sustainability Index in Australia. The environmental or sustainability index weightings are often carbon emissions per unit milk to help reduce emissions intensity, which is similar to the impact of economic index selection in national populations in recent decades^12,13,14, however this ignores the potential for improvements in gross efficiency associated with carbon emissions per cow as in the present study.

The present study showed that ranking cows on their economic and carbon index is possible and can help select more sustainable animals in the future. There was a high negative rank correlation between the economic and carbon index values for cows and the percentage of cows classed as sustainable increased during the years studied. Benchmarking cows on a carbon index per cow, along with an economic index, can help to reduce the carbon footprint of milk production with a balanced weighting on milk production and non-milk production traits (i.e. fertility, lifespan and efficiency traits).

Data

The genetic background of 1042 dairy cows from Hartpury University dairy herd, in the UK, were obtained from animals that had completed their lifespan between the years 2013 and 2022. The study used existing data and no ethical approval was required. The herd is a commercially run and milk-recorded herd. The herd consisted predominantly of autumn calving Holstein Friesian cows fed pasture, conserved silages and concentrate feed and milked twice per day. The genetic records consisted of predicted transmitting abilities (PTA) for each animal (Table 1). The PTA represents the estimated increase or reduction in a single trait that a cow will transmit to their offspring and relative to a cow with an average PTA of zero. Therefore, the PTA has great importance for breeding future herd replacements and the genetic progress of the herd and national milk production supplies. The herd studied represented a wide range of positive and negative PTAs for directly recorded common production (milk volume, milk fat and milk protein) and fitness (somatic cell count, lifespan and fertility) traits for each animal. The PTA values were obtained from genetic evaluations in April 2023, with available reliabilities for cow production trait PTAs (i.e. milk volume, milk fat and milk protein) averaging 56 (±11)% per cow and ranging from 25% to 81% per cow. Traits for milk volume, milk fat and milk protein represent the quantity and quality of milk produced by a cow. Somatic cell count is also a measure of milk quality and an indicator of the presence of white blood cells (leucocytes) associated with udder health and mastitis occurrence. Lifespan represents the days from birth to the end of productive life and fertility represents the days between each calving during the cow’s lifespan.

The economic (£ per unit; Eq. (1)) and carbon (kg CO₂-eq. per unit; Eq. (2)) coefficients attributed to each trait PTA value for each animal were obtained from a previous study¹¹ on the same herd used in the current study. Economic change was expressed in pounds sterling per unit trait and per cow, and carbon change was expressed in kilograms per carbon dioxide equivalents (CO₂-eq.) per unit trait and per cow. A bioeconomic model¹⁵ that has been developed over several years to calculate economic and carbon coefficients for biological traits and assess the effect of breeding decisions was used. The production, financial and nutritional input and output data for the herd, and averaged over the years 2017–2022, were used to describe the nutrient partitioning of a cow over its productive life. The model assumes the diet and feed intake meets the energy requirements (of herd replacements and lactating cows) for maintenance, growth, pregnancy, activity and lactation. Individual animal biological traits can be adjusted in order to test the effect on production, carbon emissions (methane and nitrous oxide) and economic metrics by adjusting single biological traits of interest such as those directly recorded as part of production in the current study (Table 1). The carbon emissions were those directly attributed to the animal and estimated from nutrition information, feed intake and waste sources to include enteric (calculated based on digestible organic matter in the diet, dietary fat and feeding level²²) and manure methane, direct and indirect nitrous oxide emissions from faeces, urine and stored or applied manure as used by the UK National Inventory methodology²³. The bioeconomic model has a Markov chain stochastic framework that describes the herd structure as 11 age groups including life prior to entering the herd and from lactations one to 10 to cover the likely lifespan of a milking cow. The herd is described as a vector of states (s) that cows occupy at a given point in time, and the vector of states at time t is multiplied by a matrix of transition probabilities (s × s) to generate a vector of states at time t + 1. The probability of a cow surviving to the next lactation (from lactation n to n + 1 and from lactation 1 to n) was dependent on survival during the current lactation. The model allows herd level data to be combined and the effect of changing cow biological traits on key production, environmental and economic metrics to be assessed. The derived economic and carbon coefficients were applied to individual animal PTA values for key traits within economic and carbon multi-trait indices, with the sum of traits providing the overall index value for each animal as

The economic index for the herd studied had a weighting of 49% on production traits and 51% on fitness traits, compared to 29% on production traits and 71% on fitness traits for the carbon index (Table 3). The carbon index notably placed more emphasis on fertility and less on milk protein yield and milk somatic cell count compared to the economic index, and therefore you would expect high ranking carbon index cows to be more fertile. Weightings for traits were similar to those used for UK national economic and carbon breeding indices (e.g. Profitable Lifetime and Envirocow²⁴), but more emphasis was placed on lifespan rather than milk production in the herd studied.

Data analysis

Spearman’s rank correlation coefficient (rho) in SPSS (Version 28; IBM Corp., Armonk, NY, USA) was used to test the association between economic and carbon index values. A sustainable animal was considered to have a positive economic and a negative carbon index value.