# The Carbon Footprint of Milk Production on a Farm
## 3 Results and Discussion
### Production Efficiency and CF  
Milk production efficiency is among the crucial factors affecting the levels of GHG emissions per product unit. A study by [PERSON] et al. [50] demonstrated a strong correlation between the gross livestock unit (GLU) ha\({}^{-1}\) and GHG emissions, especially N\({}_{2}\)O emissions originating from soil nitrification. Importantly, farms with a higher proportion of lactating cows and a higher milk yield showed a significantly lower CF value. The average milk production in the group of farms under study was 14,903 kg year\({}^{-1}\), ranging from 5040 to 69,960 kg year\({}^{-1}\), and the average milk yield per lactating cow reached 2588 kg year\({}^{-1}\). The stocking density was, on average, 2.37 GLU ha\({}^{-1}\), which also affected the levels of emissions from an area unit. According to the results obtained by [PERSON] et al. [48], Irish farms with the highest production efficiency, understood as the amount of milk per cow, not only achieved the lowest CF value but also the highest net income per unit of CO\({}_{2}\) eq emissions. [PERSON] et al. [44] also confirmed that a higher milk yield was significantly correlated with a lower CF value (correlation = \(-\)0.43) and higher production profitability (0.83). On Canadian farms, the average CF was 1.02 kg CO\({}_{2}\) eq kg\({}^{-1}\) FPCM. However, farms with higher productivity, shorter calving intervals and improved herd management achieved significantly lower emission levels. Analyses conducted in Mongolia by [PERSON] et al. [38] showed that more intensive farms with higher management efficiency, characterised by a lower age at first calving, better feed quality and a higher energy efficiency index, exhibited lower CF values. There was a strong negative correlation between the CF and the FPCM 100 kg\({}^{-1}\) cow's body weight. [PERSON] et al. [4], when analysing production systems in India, confirmed that farms with a lower milk yield (below 3500 kg FPCM year\({}^{-1}\) per cow) showed a CF value over two-fold higher than farms with a higher milk yield. The importance of the first calving, shortening of sterile periods, and optimisation of feeding was also noted in terms of emission-reducing measures. In New Zealand, [PERSON] et al. [21] analysed data from over 7000 dairy farms and demonstrated that the CF was, on average, 15% higher on farms with a milk yield exceeding 6000 kg FPCM cow\({}^{-1}\) year\({}^{-1}\) as compared with farms with a milk yield below 3540 kg. The impact of the milk yield enabled the \"dilution\" of the GHG emissions and the CF in relation to the product unit. Similar conclusions were drawn in Ethiopia, where, as indicated by [PERSON] et al. [51], over 50% of the CF variation between farms was directly attributable to differences in the milk yield. [PERSON] et al. [39], when investigating systems in Kenya, stressed that farms with feed efficiency higher than 0.34 kg\({}^{-1}\) FPCM kg\({}^{-1}\) dry feed matter achieved a significantly lower CF value than farms with lower efficiency. It should be noted, however, that increasing the supply of concentrated feed without its optimal use can lead to an increase in emissions, both direct and indirect. [PERSON] et al. [41] showed that an increase in the milk yield from 2000 to 5000 kg FPCM cow\({}^{-1}\) year\({}^{-1}\) resulted in a significant drop in the CF value. This effect, however, flattened out after exceeding the threshold of 6000 kg, indicating the existence of a limit beyond which further intensification did not bring proportional environmental benefits. It was pointed out that, with a high milk yield, the benefits of diluting the unit emissions were limited by indirect emissions from the production of feed and fertiliser and energy consumption. [PERSON] et al. [4] also demonstrated that farms with a milk yield below 3500 kg cow\({}^{-1}\) year\({}^{-1}\) showed a CF value over two-fold higher than that on farms achieving over 5000 kg. This difference resulted from low feed digestibility, poor feed ration balancing, and high nitrogen losses.