Pasture production, nutritive value and water use efficiency of irrigated dairy pasture mixtures grazed by dairy cows

Abstract

The objective of this thesis was to compare herbage dry matter (DM) production, nutritive value and water use efficiency of simple perennial ryegrass/white clover and tall fescue-white clover pastures to diverse pastures where additional grasses, legumes and herbs were added to simple grass-clover mixture. Three experiments were conducted in Canterbury, New Zealand with all pastures irrigated and grazed by dairy cows. The first study, conducted at a paddock scale, measured annual and seasonal herbage DM production, botanical composition and nutritive value of two species mixtures of perennial ryegrass (standard and high sugar) and tall fescue sown with white clover compared with more diverse mixtures where additional herbs (chicory and plantain), legumes (lucerne or red clover) and grasses (prairie grass) were added to the two species mixtures. Averaged over 2 years, annual herbage DM production was 1.6 t DM/ha greater in diverse (16.8 t DM/ha) than simple (15.2 t DM/ha) pastures, primarily reflecting greater DM production in summer. Diverse pastures had lower metabolisable energy (ME) (12.0 vs 12.2 MJ ME/kg DM) and neutral detergent fibre (301 vs 368 g/kg DM) content than simple pastures, although the total ME produced per year was greater in diverse than simple pastures (202 vs 185 GJ ME/ha). Ryegrass-based pastures had higher annual DM production (16.8 t DM/ha) than tall fescue-based pastures in the first (14.5 t DM/ha) but not second year. The second study, conducted at a small plot scale, measured annual and seasonal herbage DM production, botanical composition and nutritive value of simple and diverse pasture mixtures grazed by dairy cows subjected to full and partial irrigation. Measurements were made over two years for a simple perennial ryegrass/white clover pasture (S) and pastures with additional legumes (red clover and lucerne, SL), herbs (chicory and plantain, SH), grasses (prairie grass and timothy, SG), herbs and legumes (SLH), or herbs, legumes and grasses (SLHG) added to the simple pasture. In the partial irrigation treatment, no irrigation was applied for a 2 month period from 14 January 2011 to 10 March 2011 (Year 1) and from 7 January to 16 March 2012 (Year 2). Averaged over two years, annual herbage DM production was greater where additional legumes were added to mixtures than where additional herbs or grasses were added or in the simple mixture (16.5, 16.1, 15.1, 14.5, 14.1, 13.6 t DM/ha for SLHG, SLH, SL, SH, S and SG, respectively). The decline in DM yield associated with partial compared to full irrigation was lower in SL (10%) and SLH (14%) and SLHG (15%) than SG (19%) and S (26%) over the two year period. It was concluded that this reflected greater growth of the tap rooted legumes lucerne and red clover during the period of irrigation restriction. All pasture mixtures had similar ME content (11.1 to 11.5 MJ ME/kg DM) but mixtures containing additional legumes (SL, SLH and SHLG) had higher crude protein content (210 to 215g/kg DM) than mixtures containing additional grasses or the simple mixture (184 to 195 g/kg DM). The third study measured water use in the small plot study outlined in study two in order to investigate the production differences between full and partially irrigated pastures. Neutron probe tubes were inserted to a depth of 2.5 m and water use measured. Water use was greater in mixtures containing additional legumes (SL, SLH and SLHG, 689 mm to 705 mm) than the mixture containing additional grasses (SG, 680 mm) or the simple mixture (S, 670 mm). However, the effect was relatively small (range 14 mm to 35 mm) and was tightly linked to the summer period. In mixtures containing additional legumes (SL, SLH and SLHG), water was extracted to greater depths (0-2m) than the mixtures containing additional grasses or the simple mixture (S, SG; 0-1m). Mixtures with additional herbs (SH) extracted water to 0-1.5m soil depth. Water use efficiency (WUE) was greater in the mixtures containing additional legumes and additional herbs (SH, SL, SLH and SLHG, 18.6 to 21.1 kg DM/ha/mm) than additional grasses (SG, 17.5 kg DM/ha/mm) or the simple mixtures (S, 18.3 kg DM/ha/mm). In conclusion, the DM production and nutritive value of diverse pastures was similar or greater than that of standard perennial ryegrass/white clover pastures or tall fescue-white clover pastures. Under water restriction, DM production was less affected in mixtures containing the tap rooted legumes red clover and lucerne. Combined with the environmental benefits of diverse pastures (e.g. reduced urinary N excretion) demonstrated in other studies, it is concluded that diverse pastures are a promising alternative to perennial ryegrass-white clover pastures to deliver high production, with lower environmental implications, in dairy systems.