Abstract

The aim of this study was to investigate the use of gibberellic acid (GA) to improve feed supply and milk yield in late lactation. A grazing study at Lincoln used 36 lactating, mixed-age, Friesian-Jersey cross dairy cows balanced over three un-replicated treatments of perennial-ryegrass and white-clover pasture treated with nitrogen (N) fertiliser as urea (50 kg N/ha), GA as ProGibb® SG (8 g GA/ha) or untreated (Control). Treatments were applied in late March 2014 and grazed over 12 days in May. There was no effect of treatment on herbage pre-grazing pasture mass (2058±6.9 kg DM/ha, P=0.20), clover (3.6±0.86%, P=0.53), ryegrass (88.0±2.33%, P=0.24) or crude protein (CP) content (17.8±1.60%, P=0.31). All milk yield components: 14.6±0.40 L/day, 1.4±0.09 kg MS/cow/day, 5.4±0.33% fat and 4.4±0.21% protein were similar among treatment groups (P=0.71, 0.14, 0.16 and 0.06, respectively). Although faecal-N content of cows on GA pasture (3.72% N) was higher (P<0.001) than cows on N-fertilised pasture (3.45% N) there was no treatment effect (P>0.05) on urinary-N content (0.4 ± 0.02% N) or N excretion in milk (101±4.4 g N/cow/d). The lack of response to GA or N fertiliser highlights the importance of timing of growth promotants for late lactation. Keywords: perennial ryegrass; white clover; urea; growth promotants; dairy cow; milk production

ME, Miller, GR Edwards, H Hague, and RH Bryant

New Zealand Journal of Animal Science and Production, Volume 81, Online, 190-195, 2021