Nitrogen leaching losses from fodder beet and kale crops grazed by dairy cows in southern Southland

Abstract

Fodder beet has become increasingly common as both a winter forage and as a supplement at the shoulders of the dairy season in southern New Zealand. One advantage over the more traditional kale crop option is that fodder beet results in less urinary nitrogen (N) excretion in dairy animals, potentially reducing N leaching. Two trials were undertaken to measure nitrogen leaching losses under both autumn-grazed or autumn-lifted fodder beet crops. Leaching losses were also measured from winter-grazed fodder beet and winter-grazed kale treatments. Results from Trial 1 show that leaching losses from autumn-lifted or autumn-grazed fodder beet treatments were large (108–131 kg N ha-1) relative to losses measured in the winter-grazed fodder beet treatment (82 kg N ha-1). This indicates that autumn-grazed fodder beet crops have a greater potential for N leaching than winter-grazed fodder beet. The practice of lifting and removing fodder beet during autumn appeared to reduce N leaching somewhat, but losses were still relatively large, perhaps due to carryover of N from the previous season as a result of the dry summer conditions that preceded the drainage season in in the first year of Trial 1. The amount of N leached from the winter-grazed fodder beet treatment from Trial 1 at 82 kg N ha-1 was 50% less than the 176 kg N ha-1 observed for the kale crop. Results from Trial 2 using larger plots showed a similar trend, with winter-grazed fodder beet leaching 42% less N than winter-grazed kale (41 vs 70 kg N ha-1; P<0.001), despite not all the urine N being collected by the end of the drainage season. These losses are relatively large compared to the annual N leaching losses measured from pasture paddocks on the same farm, which ranged from 13–23 kg N ha-1. Considerations of grazing and/or harvest timing (autumn vs winter) as well as crop type appear to be important factors that determine N leaching losses from Southland dairy systems.