ABSTRACT
Irrigated, diverse pastures were managed under normal (grazed to 3.5 cm) or lax (grazed to 5 cm allowing ryegrass seed head 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 × Jersey dairy cows each were randomly allocated to three replicates of three treatments: normal 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 dry matter intake (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.
Introduction
Dairy farming, one of New Zealand’s largest industries, contributes $13.9 billion NZD of dairy exports to the New Zealand economy (Dairy NZ Citation2015). Proposed regulations in NZ are increasing the pressure on intensive pastoral dairy farming to adopt systems that increase productivity and maintain economic viability while reducing environmental footprints, including nitrogen (N) loading into the soil. The inclusion of grasses, legumes (lucerne) and herbs (chicory and plantain) in a diverse mixed sward can reduce urinary N concentration, in lactating dairy cows without negatively affecting milk production (Woodward et al. Citation2013; Edwards et al. Citation2015; Bryant et al. Citation2017) and potentially mitigate the environmental N footprint (Beukes et al. Citation2014). Compared to the conventional perennial ryegrass–white clover mixture in a New Zealand farm system, these diverse pasture swards provide a similar dry matter production and high-quality feed throughout the grazing season (Nobilly et al. Citation2013; Woodward et al. Citation2013). Moreover, grazing management regimens which increase herbage yield indicates improved soil nutrient uptake and a potential role in mitigating nutrient leaching. However, these diverse pastures may require different grazing management strategies than the conventional pasture (Lee et al. Citation2012).
In New Zealand dairy pasture systems, a post-grazing residual of 1500–1600 kg DM/ha, equivalent to 3.5–4.0 cm sward surface height, is recommended for perennial ryegrass–white clover pastures to reduce reproductive material and manage quality (Holmes et al. Citation1992; Hoogendoorn et al. Citation1992). However, others have suggested the adoption of a more lax management of 5–6 cm sward surface height (1700–1800 kg DM/ha) in the spring (Matthew et al. Citation1989; Da Silva et al. Citation1994; Hernández Garay et al. Citation1997). A lax defoliation regimen which allows the parent grass tiller to reach anthesis prior to mowing or grazing has shown to aid daughter tiller survival and improved persistence (‘late control’; Matthew et al. Citation1989; Matthew Citation1991). With the adoption of a lax grazing regimen early in the spring season, Da Silva et al. (Citation1994) reported a 20% increase in spring and summer pasture production in ryegrass swards. While this approach may improve the longevity of the ryegrass sward, there is a compromise in pasture quality due to the accumulation of stem material during anthesis. Although quality is compromised in lax-grazed ryegrass-dominant pastures (Holmes et al. Citation1992; Hoogendoorn et al. Citation1992; Da Silva et al. Citation1994), there are likely to be smaller compromises in quality in a diverse pasture with a high proportion of herbs. However, little research on the optimisation of grazing management practices for dairy cows grazing diverse pastures which contain herbs and alternative legumes has been conducted.
The use of mechanical defoliation techniques such as mowing offers a means of removing stem and accumulated dead material, which animals would otherwise reject, enabling high-quality green leaf in the subsequent regrowth (Kolver et al. Citation1999). In practice, pre-graze mowing of pastures has become a common management tactic in New Zealand dairy farm systems, even though research results using ryegrass-dominant swards have been inconsistent. One study showed that pre-graze mowing increased dry matter intake (DMI) and milk production (Bryant Citation1982), while others showed that mowing before grazing reduced DMI and milk response (Kolver et al. Citation1999; Irvine et al. Citation2010; Bryant et al. Citation2016). The explanation for variation in intake results arise from reduced selection opportunity for a higher quality diet when offered mown material (Kolver et al. Citation1999; Irvine et al. Citation2010), while others reported reduced allocation due to slow herbage regrowth following mowing (Bryant et al. Citation2016). One of the perceived benefits of mowing is improved utilisation of pasture, if all mown material is consumed. However, no tool exists which allows rapid, easy measurement of refused mown pasture. On the other hand, herbage mass tools such as the rising plate meter (RPM) will provide information on post-grazing or post-mowing residuals. Severe defoliation, such as that achieved by mowing, results in loss of leaf area which takes longer to re-establish compared with less severe defoliation (Brougham Citation1956). Response to defoliation of diverse pastures containing deep-rooted species has not been closely examined and may present some opportunities to overcome issues surrounding regrowth after mowing.
The proposed research is part of an investigation comparing the immediate and carry-over effects of grazing intensity of diverse pastures in spring on herbage production and milk yield with or without the aid of mowing (Cun et al. Citation2017). Initial results revealed lax grazing increased herbage mass but also depressed milk yield which one-off pre-graze mowing was unable to offset. The second phase of the study, proposed here, is to compare the carry-over effect, of having mown or grazed those same pastures, on pasture quality, apparent intake and milk yield and evaluate the net benefit of grazing intensity and mowing of diverse pastures in early spring.
Materials and methods
Experimental site and design
The experiment was conducted at the Lincoln University Research Dairy Farm in Canterbury, New Zealand. The experiment was a completely randomised design with three early spring grazing treatments and three replicates. The three grazing treatments were: Normal grazing (Norm, pasture consistently grazed to a compressed height of 3.5 cm); Lax grazing (Lax, pasture previously been lax grazed until perennial ryegrass reached anthesis then pasture was grazed to a compressed pasture height of 3.5 cm); and Pre-graze mowing (Mow, pasture previously been lax grazed until perennial ryegrass reached anthesis then pasture was mown to a compressed height of 3.5 cm).
The experimental area (3.0 ha) was an established pasture containing a mixture of perennial ryegrass, Lolium perenne (cv. Arrow AR1); white clover, Trifolium repens (cv. Weka); lucerne, Medicago sativa (cv. Torlesse); chicory, Cichorium intybus (cv. Choice); plantain, Plantago lanceolata (cv. Tonic). Pasture treatments commenced in early spring (September 2015) when the area was divided into two adjacent 1.5 ha paddocks using permanent fencing materials and these were further divided into 0.75 ha areas using temporary fencing materials. Norm and Lax grazing treatments were randomly allocated within each 1.5 ha area. A large mob of cows, from which experimental animals were later selected, were used to graze pastures to their desired residuals during the pasture preparation period (see for dates and pasture heights). In November, all treatment areas were grazed or mown to a residual compressed height of 5 cm to investigate the immediate effect of grazing management on milk yield and pasture production and these results are reported by (Cun et al. Citation2017). Following experiment one, the experimental cows were returned to the main mob of cows and the experimental area was left to regrow until 14 December 2015 when experiment two commenced. During the regrowth period, the area was irrigated and fertilised with urea at a rate of 45 kg N/ha.
Table 1. An initial set-up phase (September–October) in the experimental area was grazed by dairy cows to create different pasture masses to allow the area to be grazed at the same time during the trial period in November.
Animals
Animals for the second grazing study were reselected from the main mob of cows based on covariate measurements carried out 10 days prior to the study. Twenty-seven mid-lactation Friesian × Jersey dairy cows were blocked into nine groups of three cows according to milk solids production (1.78 ± 0.01 kg MS/cow/day), live weight (473.8 ± 8.6 kg), days in milk (114.6 ± 1.8 days) and age (4.92 ± 0.4 years) (all means ± SEM). Each group, containing three cows, was randomly allocated to three replicates for each treatment. Cows had ad lib access to water through a portable water trough which was shifted daily. Cows were milked twice daily (approximately 0700 and 1500 h) and offered a target herbage allowance of 35 kg DM/cow/day above ground level based on herbage mass measured with an RPM. Cows received a new allocation daily following the afternoon milking. Herbage allocation during the experiment was based on pasture mass as estimated daily by RPM compressed height which was recorded daily pre- and post-grazing. Because the pastures consisted of 60% ryegrass and clover, the RPM manufacturers calibration (i.e. kg DM/ha = 140 × RPM reading + 500, where an RPM reading = 0.5 cm unit) was used to determine grazing area allocation.
Herbage measurements
Herbage mass was determined retrospectively from calibration harvests collected throughout the study. To calibrate the RPM, a 0.2 m2 quadrat was placed randomly onto the pasture and two RPM measurements were recorded in the quadrat area. All herbage was harvested to ground level, washed, oven dried and weighed. Best-fit regression between RPM and harvested DM yield was used to derive calibrations for each treatment:
Apparent DMI was determined from herbage DM disappearance between pre-graze mass and post-graze mass.
Herbage samples for nutritive analysis were collected daily by cutting standing pasture to ground level from random locations (n = 15) pre- and post-grazing in each daily herbage allocation. Samples were mixed and divided into two subsamples. One subsample of approximately 100 g was freeze-dried and passed through a 1 mm sieve (ZM200 rotor mill Retsch Inc., Pennsylvania, USA) and analysed for chemical composition and in vitro organic matter digestibility in the dry matter (DOMD) using near-infrared spectroscopy (NIRS; Model: NIRSystems 5000, Maryland, USA) by the Lincoln University Analytical Laboratory and had been calibrated using pasture species similar to those in this experiment. Metabolisable energy (ME) was calculated as ME MJ/kg DM = 0.16 × %DOMD (McDonald et al. Citation2010). The second fresh subsample of approximately 100 g was hand sorted into perennial ryegrass, herbs, legumes, weeds and dead material. Samples were then oven dried at 60°C for 48 h and weighed and the botanical composition percentage was determined on a DM basis.
Accumulated herbage mass for the entire period was the sum of the difference between pre- and post-graze mass at each grazing event between September and December. Estimation of herbage mass accounted for the change in sward structure over time using three sets of calibrations for each treatment. During the preparation period from September to October, herbage mass (kg DM/ha) was estimated using the RPM manufacturers equation. Herbage mass in November was calculated using equations developed in experiment one (Cun et al. Citation2017). Herbage mass in December was determined using calibrations from the present trial stated above.
Animal measurements
Milk yield was recorded daily at both morning and afternoon milking for individual cows with an automated system (DeLavel Alpro Herd management system, DeLavel, Tumba Sweden). Milk samples were collected on days 0, 5, 6, 7 and 8 of the experiment. Samples were analysed by Livestock Improvement Corporation Ltd (LIC; Christchurch, New Zealand) to determine milk fat, protein and lactose by MilkoScan (Foss Electric, Hillerød, Denmark).
Statistical analysis
The effect of grazing management on the botanical composition, nutritive value, pasture regrowth, herbage mass, DMI, milk production and milk composition was analysed by a one-way analysis of variance (ANOVA) with three replicates (GenStat 15.1, VSN International Ltd, 2012). Pooled means for animals and days were used in the analysis which was carried out on nine experimental units (3 treatments × 3 replicates). Means were separated using Fisher’s protected least significant difference test whenever ANOVA indicated a significant treatment effect.
Results
There was no treatment effect on botanical composition. The most dominant species was perennial ryegrass which accounted for half the biomass with respective proportions of legumes, herbs, dead material and weeds at 22.4%, 18.7%, 8.8% and 0.16%, of the DM ().
Table 2. Botanical composition (% of DM in pre-grazing mass) of Norm, Lax and Mow treatments in December.
In the previous rotation, grazing and mowing treatments achieved a compressed post-grazing height of 5 cm which did not differ between treatments. Subsequent herbage growth rates, herbage mass or herbage chemical composition were not affected by previous grazing treatment (). Additionally, there were no differences in post-grazing herbage mass nor herbage chemical composition among treatments (). The accumulated yield for the spring period (September to December) was greatest (p < .01) for lax-managed pastures (11,657 ± 284 kg DM/ha) compared to Mow (10,751 ± 202 kg DM/ha) or Norm (10,574 ± 79 kg DM/ha).
Table 3. Pasture regrowth, pre- and post-grazing herbage mass, pre- and post-grazing forage dry matter (DM), acid detergent fibre (ADF), water-soluble carbohydrates (WSC), digestible DM in organic matter (DOMD), neutral detergent fibre (NDF), organic matter (OM), organic matter digestibility (OMD), crude protein (CP), metabolisable energy (ME) of Norm, Lax and Mow treatments.
Using herbage mass data derived during the experiment, the actual daily herbage allocation was calculated as 34.9 ± 0.13 kg DM/cow/d for all treatments. Apparent DMI was unaffected by pasture treatments. Milk yield (20.1 kg/cow/d) and milk solids (1.85 kg MS/cow/d) was unaffected by treatment (). Overall, there was no effect of grazing management on average milk yield across the two grazing events in experiments one (Cun et al. Citation2017) and two where combined milk yields for Norm, Lax and Mow were 45.89, 46.26, 45.38 ± 0.36 kg/cow, respectively (p = .32). Also, grazing management had no effect on milk solids across the two grazing events where combined average milk solids for Norm, Lax and Mow were 4.23, 4.21, 4.12 ± 0.04 kg MS/cow, respectively (p = .28).
Table 4. Mean milk yield, milk composition and estimated DMI of Norm, Lax and Mow treatments.
Discussion
This experiment was designed to test the carry-over effects of a one-off pre-graze mown diverse pastures on nutritive value, DMI and milk solids yield of mid-lactation dairy cows. It was hypothesised that a one-off pre-graze mown pastures in the previous grazing rotation would result in an improved nutrient intake, increase apparent DMI and greater milk solids yield at the subsequent grazing, compared to treatments that only used grazing to control herbage mass. However, in this experiment, grazing management treatments resulted in similar nutritive value and no difference in DMI and consequently no change in milk yield when herbage was offered at the same allowance.
Both Bryant (Citation1982) and Kolver et al. (Citation1999) concluded that pre-graze mowing of ryegrass and white clover pastures resulted in improved milk yields of at least 5% in the subsequent grazing due to improved pasture quality and more uniform grazing. The lack of treatment effect in the current study might be explained by similar post-defoliation sward heights from the previous grazing (5 cm) resulting in similar regrowth characteristics of the herbage and no differences in herbage quality.
Due to the large proportion of dead material and reproductive stem content on pasture-based systems in the summer, energy is often the most limiting nutrient for dairy cows. However, the metabolisable energy content of these diverse pastures was greater (>11.3 MJ ME/kg DM) than that found by Kolver et al. (Citation1999) in their ryegrass–white clover pastures for the summer period (9–10 MJ ME/kg DM). A diverse pasture is able to maintain a high nutritive value (Ulyatt et al. Citation1976; Barry Citation1998) and possibly alleviate the magnitude of low energy levels relative to ryegrass. Kolver and Muller (Citation1998) reported a greater milk production associated with a greater nutrient intake. With the inclusion of alternative legumes and herbs, pasture quality is generally improved to possibly offset the nutritive value and perhaps no great effects in milk solids production.
There was no statistical difference in herbage growth rates or pre-graze herbage mass, though pre-graze herbage mass of mown pastures was numerically (200 kg DM/ha) lower than the conventional ‘Norm’ grazing treatment. Cows in mown treatments were also able to achieve a numerically lower post-grazing residual compared with grazed only treatments which maintained a consistent DM intake across treatments. This raises questions around the cumulative effect of frequent mowing on herbage regrowth and pasture quality. In this study, a one-off mowing to return pastures to an acceptable residual resulted in similar regrowth between treatments in a single rotation, but over the entire period from September to December, accumulated pasture growth was similar for Mow and Norm and greatest for lax-managed pastures.
In this experiment, the combination of similar post-grazing (5 cm) from the previous grazing rotation and a diverse pasture mixture, an adequate supply of herbage with an adequate nutritive value was available and possibly explain why milk production was similar across all treatments. The plant nutritive value in these diverse pastures may not be of a sufficient magnitude to affect spring management practices.
Conclusion
This study showed that if grazing or mowing achieved similar post-grazing residuals in the previous grazing event, the subsequent quality of regrowth of diverse pastures is unlikely to be affected by early spring grazing management. The similarity in herbage quality results in similar milk yield when offered at the same allowance. It has been demonstrated in the current study and our previous study, diverse pastures can be maintained to a greater herbage mass with reproductive development in spring without affecting milk solids production when grazed in the summer.
Acknowledgements
The Forages for Reduced Nitrate Leaching programme is a partnership between DairyNZ, AgResearch, Plant & Food Research, Lincoln University, the Foundation for Arable Research and Landcare Research. The authors gratefully acknowledge staffs and students at Lincoln University Research Dairy Farm who provided technical assistance.
Disclosure statement
No potential conflict of interest was reported by the authors.
Additional information
Funding
References
- Barry T. 1998. The feeding value of chicory (Cichorium intybus) for ruminant livestock. The Journal of Agricultural Science. 131(03):251–257. doi: 10.1017/S002185969800584X
- Beukes PC, Gregorini P, Romera AJ, Woodward SL, Khaembah EN, Chapman DF, Nobilly F, Bryant RH, Edwards GR, Clark DA. 2014. The potential of diverse pastures to reduce nitrogen leaching on New Zealand dairy farms. Animal Production Science. 54(12):1971–1979.
- Brougham R. 1956. Effect of intensity of defoliation on regrowth of pasture. Australian Journal of Agricultural Research. 7(5):377–387. doi: 10.1071/AR9560377
- Bryant AM. 1982. Effects of mowing before or after grazing on milk production. In: Macmillan KL, Taufa VK, editor. Dairy production from pasture. Hamilton, NZ: Clark and Matheson, Ltd.; p. 381–382.
- Bryant R, Kingsbury L, Edwards G. 2016. Does mowing before grazing increase dry matter intake and milk yield? Proceedings of the New Zealand Society of Animal Production. 76:73–77.
- Bryant RH, Miller ME, Greenwood SL, Edwards GR. 2017. Milk yield and nitrogen excretion of dairy cows grazing binary and multispecies pastures. Grass and Forage Science. 72(4):806–817. doi: 10.1111/gfs.12274
- Cun GS, Edwards GR, Bryant RH. 2017. The effect of pre-graze mowing on milk production of dairy cows grazing grass–herb–legume pastures managed under contrasting spring defoliation regimes. Animal Production Science. 57(7):1414–1418. doi: 10.1071/AN16458
- Da Silva SC, Hodgon J, Matthews P, Matthew C, Holmes C. 1994. Effect of contrasting spring grazing management on summer-autumn pasture and milk production of mixed ryegrass-clover dairy swards. Proceedings of the New Zealand Society of Animal Production. 54:79–82.
- Dairy NZ. 2015. Dairy NZ Annual Report 2014/15. Strategy for sustainable dairy farming 2013–2020. Hamilton: Dairy NZ.
- Edwards GR, Bryant RH, Smith N, Hague H, Ferris A, Farell L. 2015. Milk production and urination behaviour of dairy cows grazing diverse and simple pasture. Proceedings of the New Zealand Society of Animal Production. 75:79–83.
- Hernández Garay A, Matthew C, Hodgson J. 1997. Effect of spring grazing management on perennial ryegrass and ryegrass-white clover pastures. New Zealand Journal of Agricultural Research. 40(1):37–50. doi: 10.1080/00288233.1997.9513228
- Holmes CW, Hoogendoorn CJ, Ryan MP, Chu ACP. 1992. Some effects of herbage composition, as influenced by previous grazing management, on milk production by cows grazing on ryegrass/white clover pastures. 1. milk production in early spring: effects of different regrowth intervals during the preceding winter period. Grass and Forage Science. 47(4):309–315. doi: 10.1111/j.1365-2494.1992.tb02276.x
- Hoogendoorn CJ, Holmes CW, Chu ACP. 1992. Some effects of herbage composition, as influenced by previous grazing management, on milk production by cows grazing on ryegrass/white clover pastures. 2. Milk production in late spring/summer: effects of grazing intensity during the preceding spring period. Grass and Forage Science. 47(4):316–325. doi: 10.1111/j.1365-2494.1992.tb02277.x
- Irvine L, Freeman M, Rawnsley R. 2010. The effect of grazing residual control methods on cow intake and milk production in late spring. Proceedings of the Australasian Dairy Science Symposium. 4:195–198.
- Kolver E, Penno J, MacDonald K, McGrath J, Carter W. 1999. Mowing pasture to improve milk production. Proceedings of the New Zealand Grassland Association. 61:139–145.
- Kolver ES, Muller LD. 1998. Performance and nutrient intake of high producing Holstein cows consuming pasture or a total mixed ration. Journal of Dairy Science. 81(5):1403–1411. doi: 10.3168/jds.S0022-0302(98)75704-2
- Lee J, Hemmingson N, Minnee E, Clark C. 2012. Chicory and plantain management strategies to increase herbage dry matter yield, nutritive value and plant survival. Proceedings of the 5th Australasian Dairy Science Symposium. 5:393–397.
- Matthew C. 1991. “Late Control” – what is it and why should it work? Dairyfarming Annual. 43:37–42.
- Matthew C, Xia J, Hodgson J, Chu A. 1989. Effect of late spring grazing management on tiller age profiles and summer-autumn pasture growth rates in a perennial ryegrass (Lolium perenne L.) sward. Proceedings of XVI International Grassland Congress. 16:521–522.
- McDonald P, Edwards R, Greenhalgh J, Morgan C, Sinclair L, Wilkinson R. 2010. Animal nutrition, 7th ed. London: Pearson Prentice Hall.
- Nobilly F, Bryant R, McKenzie B, Edwards G. 2013. Productivity of rotationally grazed simple and diverse pasture mixtures under irrigation in Canterbury. Proceedings of the New Zealand Grassland Association. 75:165–172.
- Ulyatt M, Lancashire J, Jones W. 1976. The nutritive value of legumes. Proceedings of the New Zealand Grassland Association. 38:107–118.
- Woodward S, Waugh C, Roach C, Fynn D, Phillips J. 2013. Are diverse species mixtures better pastures for dairy farming. Proceedings of New Zealand Grasslands Association. 75:79–84.