On conventional dairy farms, the N-surplus per unit of N in produce decreased with increasing milk yield per cow.
Also, the N-surplus per unit of N-produce was on average lower on organic than on conventional farms, 4.2 ± 1.2 kg N·kg N− 1 and 6.3 ± 0.9 kg N·kg N− 1, respectively.
An increase in N-input per hectare increased the amount of N-produce in milk and meat per hectare, but, on average for all farms, only 11% of the N-input was utilised as N-output; however, the N-surplus per unit of N in produce (delivered milk and meat gain) was not correlated to total N-input.
The N-surplus per hectare was calculated as N-input (net N-purchase and inputs from biological N-fixation, atmospheric deposition and free rangeland) minus N in produce (sold milk and meat gain), and the N-surplus per unit of N-produce as net N-input divided by N in produce.
This generally led to the average hill farm having the highest product-related environmental impacts, but this system had the lowest nutrient surpluses per unit area.
Since surplus production is the outcome of invested leaf number, area, dry mass and N, leaf efficiency defined as surplus production per unit investment of these leaf variables should be an important parameter of leaf activity in the canopy.
Leaf productivity is the surplus production per unit standing leaf variable per unit time [ eqn (8)].
We define 'leaf productivity' as a surplus production per unit leaf variable (g [leaf variable]−1 d−1), together with their MRT to calculate 'leaf efficiency' (g [leaf variable]−1; Hirose and Oikawa, 2012).
Production cost (per unit input).
This study was designed to analyse the important variables influencing nitrogen (N) surplus per hectare and per unit of N in produce for dairy farms and dairy systems across 10 certified organic and 10 conventional commercial dairy farms in Møre og Romsdal County, Norway, between 2010 and 2012.
This has been supported by a report published by the National Renewable Energy Laboratory NRELL) which stated that under a controlled environment, microalgae are capable of producing 40% surplus oil for biodiesel per unit area of land as compared to crops such as palm seed or sunflower.
