Brassicas (represented mainly by forage rape, kale, swede and turnip) and fodder beet are widely used in New Zealand grazing systems due to their high DM yield and high nutritional value compared to conventional perennial ryegrass/white clover pastures. Brassica and fodder beet forages are commonly used in New Zealand during periods of feed shortage through the summer, autumn and winter; to supplement periods of low pasture quality; to finish stock; as a summer-safe feed; and often to control weeds prior to pasture renewal. The aim of this review was to assess available literature on the effects of feeding brassica and fodder beet forages to livestock on methane and nitrous oxide (N2O) emissions. The focus of reducing one pollutant, such as the more efficient transfer of dietary nitrogen (N) into milk or meat and reduction of N excretion into faeces and urine and, therefore, N2O emission, may have implications for emissions of other pollutants, such as methane arising from the fermentation of feedstuffs within the ruminant forestomach. Thus, when it comes to feeding brassica and fodder beet forages to ruminant livestock, the trade-off between enteric methane production and N excretion (and therefore N2O emission) needs to be understood, both at the animal scale and at the whole farm level.
Summarising the available literature, brassicas are characterised by lower fibre (NDF; neutral detergent fibre) and higher non-structural carbohydrate concentrations, compared to traditional perennial ryegrass/white clover pastures. This marked difference in chemical composition may partly explain the average decrease of 30% in methane yield (g/kg dry matter intake [DMI]) across a range of studies. The effect of brassicas on N2O emissions from grazing systems (as measured by using ‘emission factors’ e.g. EF3) is unclear. It is difficult to interpret the effect of brassicas on N2O emissions in experiments with confounding variables, such as the application of urine (or faeces) from brassica-fed animals on soils growing conventional pasture (or vice versa of urine from pasture-fed animals applied to soils growing brassicas). There are many variables (including season, soil type, soil temperature, soil moisture, soil compaction, urine composition and volume applied etc.), that need to be accounted for in future experimentation. These factors are important as generally brassicas are used as a summer feed when the ground is warm and dry or strip grazed in winter when the soils are wet and cold and there is the possibility of pugging.
The limited data on the effects of feeding brassicas on N2O emissions together with the limited data on the amounts, yields and times of the year different brassicas are grown currently restricts our ability to assess the value of including brassicas in the New Zealand Inventory model.