Pasture grass species employed within 1944 dairy production included Kentucky bluegrass, timothy, and orchard grass (Cardon et al., 1939) with reduced protein contents than modern varieties (Huffman, 1939). Consequently, N intake per animal was considerably less and the index for N excretion somewhat less than would be predicted from the extrapolation of animal numbers from 1944 to 2007 (Table 2). Despite the capacity of the 1944 system to have a greater transfer of nutrients (N and P) into groundwater, it is interesting to note that historical manure management practices had a slight mitigating effect upon CH4 production. This is directly attributable to differences in manure storage; according to IPCC (2006), the 1944 production system, with cows spending equal time grazing and housed, would have an average methane conversion factor of 2.75% of excreted N (1.5% while grazing and 4.0% for solid manure storage) compared with 21.7% for modern lagoon-storage. However, this advantage was negated by the 1944 population size, which resulted in increased total production of CH4 and N2O from enteric fermentation and manure.