|dc.description.abstract||Strategies to increase herbage dry matter (DM) production and quality while reducing environmental impacts are sought for dairy farming systems. Two strategies to improve DM production and quality are grazing management and the choice of forage species mixture. While grazing management rules are well developed, particularly timing and intensity, for perennial ryegrass-white clover pastures, the rules are less clear for diverse pasture mixtures that include perennial ryegrass, Italian ryegrass, alternative legume species (red clover and lucerne) and herbs (chicory and plantain). This thesis examined the effect of grazing management strategies in spring and autumn on herbage DM production, botanical composition, herbage quality, and milk production of lactating dairy cows grazing diverse pastures. Four field experiments were conducted in Canterbury, New Zealand on irrigated diverse pastures grazed by dairy cows. Data were used in the FARMAX farm systems model to simulate the effects of grazing management on herbage DM production and farm profitability.
The first experiment measured over two years herbage DM production, quality and botanical composition of two diverse pasture mixture types managed either by conventional grazing by dairy cows or by lenient grazing in either spring or autumn. Diverse pasture mixture consisted of perennial ryegrass, white clover, red clover, chicory and plantain (diverse) or the same pasture mixture plus Italian ryegrass (diverse + Italian). The two pasture types were grazed in three specific regimes: (1) conventional hard grazing, where cows grazed to a compressed pasture height of 3.5 cm year-round, (2) autumn lenient grazing, where cows grazed to a compressed pasture height of 5 cm during autumn before a switch to 3.5 cm for the remainder of the year and (3) spring lenient grazing, where cows grazed to a compressed pasture height of 5 cm during spring before a switch to 3.5 cm for the remainder of the year. Annual herbage DM production was greater in diverse mixtures (13.4 ± 0.25 t DM/ha, P<0.01) than diverse pasture + Italian ryegrass (12.8 ± 0.25 t DM/ha). Averaged over two years, pastures managed by autumn lenient grazing had the lowest annual herbage DM production (12.6 ± 0.3 t DM/ha) compared to those managed by spring lenient grazing (13.8 ± 0.3 t DM/ha) or conventional grazing (13.9 ± 0.3 t DM/ha, P=0.03). Although autumn lenient grazing resulted in the lowest herbage DM production, this grazing management had a greater proportion of red clover (16.5 ± 0.9%, P≤0.01) and plantain (21.8 ± 1.4%, P≤0.05) in the first year compared to the proportion of red clover and plantain in spring lenient grazing (13.2 ± 0.9% and 20.7 ± 1.4%) or conventional grazing (12.3 ± 0.9% and 16.8 ± 1.4%). Grazing management did not affect crude protein concentration, ranging from 165 to 169 g/kg DM. Total ME produced per hectare averaged 147 GJ/ha/year and was unaffected by grazing management.
The second experiment examined the effect of five defoliation intensities (defoliation heights) during the late autumn on winter and early spring herbage dry matter production, regrowth, botanical composition and nutritive value of a diverse pasture mixture containing perennial ryegrass, Italian ryegrass, white clover, lucerne, chicory and plantain. In late autumn, pastures were defoliated to five post-grazing heights (20, 30, 40, 50, 60 mm), and herbage DM production and nitrogen concentration were measured over a 112 d regrowth period. Accumulated herbage DM production was similar across all defoliation heights when measured to ground level, ranging from 1612 to 2476 kg DM/ha, averaging 2079 ± 807 kg DM/ha. After 112 days, increasing defoliation height tended to result in greater herbage DM mass above a simulated spring grazing height of 35 mm. Contrasts between low and lenient defoliation treatments confirmed greater herbage DM accumulation from lenient autumn defoliation (2408 vs 1860 ± 262 kg DM/ha). At the initial defoliation treatment (day 0), botanical composition did not differ among defoliation heights, except for perennial ryegrass (P<0.001). At the end of the regrowth period (day 112), all treatments remained relatively stable in botanical composition except for a trend for an increased proportion of Italian ryegrass (P=0.09) in the severe (≤40 mm) defoliation treatments. Nitrogen concentration was greatest in severe defoliation (20 mm) compared to lax defoliation (60 mm) (2.88 versus 2.41%DM, respectively). However, when herbage N concentration was multiplied by the final herbage DM mass to estimate herbage N uptake, the average herbage N uptake was 40.8 kg N/ha and was similar across all treatments (P=0.99). Nutritive value (CP, ME, WSC, DOMD) was not affected by defoliation height at the final harvest, where the ME ranged from 12.0 to 12.3 MJ ME/kg DM and CP averaged 16.4% DM. It is concluded that diverse pasture mixtures can maintain its high quality through winter and defoliation height had no significant effect on herbage DM production, botanical composition or N uptake. Leaving a greater defoliation height in autumn increased the harvestable DM mass for early spring.
The third experiment examined the effect of spring grazing management with pre-graze mowing on milk production of dairy cows grazing pastures containing perennial ryegrass, white clover, chicory, plantain and lucerne. Diverse pastures were managed over two grazing rotations in spring under conventional (grazed to 3.5 cm) or lax management (grazed to 5 cm, allowing early ryegrass seedhead development before conventional grazing at anthesis ‘late control’). On the third grazing rotation, a milk production study was conducted. Thirty-six, mid-lactation spring calving Friesian × Jersey dairy cows were allocated to nine groups of four cows and randomly allocated to three replicates of the following three treatments: (1) conventional grazing (Norm), (2) lax grazing of standing herbage (Lax) and (3) lax grazing with pre-graze mowing of herbage (Mow). Cows were offered a daily herbage allocation of 30 kg DM/cow above ground level, with milk production measured over 8 days. Pastures managed under lax management had higher pre-grazing herbage mass (4149 kg DM/ha) than did pastures managed under conventional management (3105 kg DM/ha), but all treatments had similar metabolisable energy (~12.26 MJ ME/kg DM). Daily milksolid (MS) production tended to be lower (P=0.07) for cows grazing pastures managed under Lax and Mow (2.34 and 2.24 MS/cow/day respectively) than with Norm (2.43 MS/cow/day). Although there was no difference in daily MS production between mowing and greater pre-graze herbage mass, switching from a high to low post-grazing height managed by either grazing or mowing in late spring is likely to have a negative impact on milk production.
The fourth experiment compared the immediate and carry over effects of the defoliation treatments in the third experiment on milk production of dairy cows. Irrigated, diverse pastures were managed under conventional (grazed to 3.5 cm) or lax (grazed to 5 cm allowing ryegrass seedhead development) grazing intensity, with or without mowing (to 3.5 cm), in spring. On the subsequent grazing rotation in summer, an experiment was conducted to investigate the carry-over effects of previous management on herbage regrowth and milk production. Nine groups of three Friesian x Jersey dairy cows each were randomly allocated to three replicates of three treatments: conventional grazing (Norm); previously lax managed pastures (Lax); previously lax managed pastures that were pre-graze mown (Mow). Herbage in Mow treatments had a higher ME (P<0.05) than Lax and Norm (11.7, 11.3 and 11.4 MJ ME/kg DM, respectively). There was no difference in DMI (18 ± 0.30 kg DM/cow/d) or MS production (1.85 ± 0.02 kg MS/cow/d) among treatments. Results of this study indicated that milk production was not altered by grazing management.
The final study used the commercial modelling tool, Farmax Dairy Pro to assess the effect of diverse pasture mixtures and grazing management on profitability of irrigated Canterbury dairy farms. Herbage quality data and herbage DM production were used from the first experiment. The data were fitted to a base model farm (average of North Canterbury region) using Farmax Dairy Pro to produce six different farm scenarios. Farm scenarios were ranked and compared by profit expressed as earnings before tax. Farm scenarios 1-3 were diverse pasture managed by conventional grazing, autumn lenient grazing and spring lenient grazing management, respectively and farm scenarios 4-6 were diverse pasture plus Italian ryegrass managed by conventional grazing, autumn lenient grazing and spring lenient grazing management, respectively. Pastures managed by autumn lenient grazing had the lowest herbage DM production, more supplement purchased, and hence lowest profit compared with conventional or spring lenient grazing management. The diverse pasture managed by spring lenient grazing resulted in greater profit ($2,658/ha) than the other scenarios (average $2,261/ha). The greater profit was driven by greater annual herbage DM production per hectare (15.0 vs. 13.3 t DM/ha/year) in diverse pasture management by a lenient grazing in spring and, hence, less purchased feed required to meet animal demand (NZ$0/ha) compared to other scenarios (NZ$404/ha). In addition, the lenient grazing of diverse pasture in spring was the only scenario that resulted in a surplus feed supply ($3145/year). An excess of 15 t DM of pasture silage was sold in May and less purchased feed required, decreasing the operating costs/MS. Surplus feed supply was calculated based on cost to cut silage and priced to sell at $40/ha (Farmax default value for the Canterbury region). When diverse pasture is considered, spring lenient grazing is a potential management option for an irrigated Canterbury dairy farm system to increase DM production and thereby profitability.
This thesis highlights seasonal grazing management strategies of alternative diverse pasture mixtures that includes legumes (red clover and lucerne), herbs (chicory and plantain) and Italian ryegrass in a perennial ryegrass white clover mixture onto an irrigated dairy farm system with rotational grazing. The study also determined whether or not the potential production benefits would occur on farm would be profitable. The results from these projects provide relatively simple grazing management practices of diverse pasture mixtures for dairy farmers to implement into their farm systems to increase herbage DM production, maintain pasture quality with no detrimental effects on milk production. This study confirmed high DM production and quality of diverse pastures. It was demonstrated that spring grazing management (e.g., lenient grazing in spring until perennial ryegrass reached seedhead development, followed by a hard-conventional grazing) could be used to improve DM production with little effect on herbage quality. Milk production was unaffected by the lenient grazing in spring and not improved by mowing. Combined with the environmental benefits of diverse pastures (i.e., reduced urinary N excretion) demonstrated in other studies, this study confirms the role of diverse pastures in promoting environmentally sustainable dairy systems.||en