But farmers should always use the growth promotant with nitrogen and might also run the risk of post-gibberellic decline, AgFirst Northland consultant Chris Boom said.
He added that the jury was still out on multiple applications of gibberellic acid, which he called “debatable” until further trial work was done.
“Make sure you have enough leaf cover to absorb the gibberellic acid when applied, but don’t let the pasture get too long,” he advised more than 100 farmers who attended the annual conference of the Northland Dairy Development Trust.
He thanked the sponsors of the trust for funding the project.
On the Northland Agricultural Research Farm (NARF) at Dargaville, over winter and spring 2014, the trial consisted of single and repeat applications of gibberellic acid, with or without nitrogen.
Boom said the trial was conceived by Kerry Chestnut, a Whangarei dairy farmer and former farm adviser, while NARF farm manager Karla Frost was also assisted by Duncan Bayne and Stacey Belton.
Seven treatments were replicated five times on 4m by 1.5m plots:
• Control.
• Liquid urea at 37kg/ha nitrogen (N) in June, July and August.
• Gibberellic acid (GA) as ProGibb at 20g/ha with surfactant in June and July.
• Granular urea at 37kg/ha N plus GA with surfactant in June and July.
• Liquid urea at 37kg/ha N plus GA with surfactant applied in June, July and August.
• Liquid urea at 37kg/ha N applied in July.
• Liquid urea at 37kg/ha N plus GA with surfactant in July.
The single applications of nitrogen and GA, with nitrogen in July, showed the first harvest gave significantly greater responses to both nitrogen (+426kg/ha drymatter (DM)) and nitrogen with GA (+921kg DM/ha), compared with the control.
But the differences at the second harvest were not statistically significant.
The GA effectively doubled the drymatter grown and by using a GA cost of application at $38/ha, Boom showed the extra pasture gained cost 7c/kg DM, while the nitrogen-only treatment was 12c.
However, the parts of the trial concerning repeat applications of GA and nitrogen were not clear-cut.
‘Other research suggests that the best time to graze GA pastures is three to four weeks after application as yield responses do not change or decrease after five to six weeks.’
Pasture production was higher than expected during the first response period and the first harvest (July 18) was delayed because of a flood, so all treatments were approaching maximum yield of ceiling level at the time.
“This likely compromised the response of the nitrogen and GA treatments relative to the controls,” Boom said.
“Other research suggests that the best time to graze GA pastures is three to four weeks after application as yield responses do not change or decrease after five to six weeks.
“Overall, GA on its own showed no greater pasture production than the controls at any stage (five harvests between July and November), which supports the recommendation it should be applied with nitrogen.
“There was significant post-GA depression at the third, fourth and fifth harvests, ranging from -200 to -340kg/ha DM.
“Other studies also found this effect, where GA causes more erect pastures and less tillering, grazing has more severe effect resulting in less leaf area and reduced regrowth post-grazing.
“Because GA utilises plant nitrogen and carbon, this trial along with others suggests that the best response to GA occurs when nitrogen is not limiting.”
Nitrogen with GA had significantly greater pasture production than nitrogen only in harvest three (September 18) but significantly lower in harvest four (October 16), probably because of post-GA depression.
Both treatments were significantly greater than the control (+900kg/ha DM) for harvest three and four, so that overall response was assumed to be more because of nitrogen than GA.
The trial was carried on to a fifth harvest (mid-November) but all treatments were statistically similar except GA only, which was significantly lower.
Boom said multiple applications of GA with nitrogen changed the timing of pasture growth but had little effect on the total amount of pasture produced.
“Based on this trial, repeat applications of GA did not provide reliable responses, however, trial conditions may have compromised these responses,” he concluded.
