The effect of different irrigation strategies on turnip forage crop growth rates, dry matter (DM) yield, water use efficiency (WUE), changes in soil volumetric water content, nutritive characteristics and mineral content was determined on different soil types at different sites (site 1 and 2) over 2 years. Treatments were: (A) a dryland control; (B) fully watered to soil field capacity each week; (C) 75% of full watering; (D) 50% of full watering; (E) 25% of full watering; (F) a single watering to soil field capacity or to a maximum of 50 mm between weeks 0–6; (G) a single watering between weeks 6–8; (H) a single watering between weeks 8–10; and (I) a single watering between weeks 10–12 after sowing. In addition, each irrigation treatment received either 0 or 50 kg N/ha applied 5 weeks after sowing.
Responses to applied irrigation water were different at each site and also within one year. At site 1, responses to irrigation were adversely affected by insect damage and delayed sowing, particularly in year 1. However, there were significant increases in DM yield to weekly irrigation regimes in both years, with responses greater in year 2, and responses in both years were greater where nitrogen was applied. At site 2, there were significant responses to weekly irrigation regimes in year 1 with DM yields from fully irrigated plots almost double that of the dryland treatment. In year 2, DM yields from all treatments were similar and it is proposed that lower summer temperatures may have contributed to the improved DM yield observed with the dryland treatment. In both years, at site 2, there were generally higher DM yields with nitrogen application irrespective of irrigation regime. Turnip metabolisable energy values were consistently above 11.5 and 13 MJ/kg DM for leaves and roots respectively, with crude protein contents for leaves ranging from 11 to 20% and 13 to 24% and roots from 6 to 14% and 9 to 17% at sites 1 and 2, respectively.
Water use efficiencies varied according to irrigation treatment with higher efficiencies observed at site 2 in both years. In year 1 and 2, total WUE at site 1 varied from 5 to 11 kg DM/ha.mm while at site 2 the range was 20–48�kg�DM/ha.mm with higher values being observed in year 2. As with DM yields it is likely that the observed higher WUE in year 2 was due to lower summer temperatures. At site 2, the dryland treatments produced the highest efficiencies in both years. In contrast, WUE from applied irrigation water ranged from 0 to 35 kg DM/ha.mm at site�1 and from 0 to 23 kg DM/ha.mm at site 2.
This study suggests that there is potential to economically irrigate turnips to provide additional DM of high nutritional value for lactating dairy cows, however, issues such as sowing dates, soil type, and insect damage will also influence final yields. In particular, summer temperatures influence both dryland growth potential and growth responses to irrigation. Also single irrigations during the growing period will not significantly increase DM yields over a crop grown under dryland conditions.
Irrigated dairy pasture yield and water use efficiency responses to summer applied nitrogen