by Simon Moloney (consulting agronomist)

Pasture Renewal Systems

Profitability: Costs will vary depending on the method of pasture renewal selected (oversowing – low, to full cultivation – high). The profitability will depend on the degree of establishment and subsequent pasture management. There are a lot of variables, many within the farmer’s control and some key variables, like weather, out of their control. Typically, internal rates of return (IRR) on renewal range from 15 to 50%+.

Benefits: New pastures will increase annual DM yield, provide additional seasonal yield (early-spring growth), increase pasture quality (higher digestible organic matter – DOM, higher ME) leading to higher voluntary feed intake (VFI), greater pasture utilisation, increased MS production and reduced GHG emissions.

Pasture Renewal Techniques

Oversowing: Lowest cost normally with the highest variability in success due to competition from resident pasture species and the likelihood of variable rainfall distribution. Most pasture species respond well to oversowing including: chicory, plantain, clovers and ryegrasses, especially hybrid and Italian ryegrasses.

Undersowing: The use of a direct drill without a non-selective herbicide. Paddocks need to be hard grazed prior to drilling with covers managed carefully following establishment. Like oversowing, this option is best done in the early autumn where soil moisture is likely to become more reliable, but before soil temperatures decline. Both techniques have minimal impact on cow carrying capacity (CCC) allowing larger proportions of the farm to be sown annually (10-25%). Owning your own drill can definitely improve timing and flexibility. Ensure that seed placement depth does not exceed 20 mm (aim for 15 mm) with small seed species (clovers, forage herbs) all sensitive to sowing depth. This system suits the high seedling vigour associated with modern perennial and hybrid ryegrass cultivars. Both techniques will encounter a degree of competition from old resident pasture species ultimately impacting on seedling establishment percentage.

Spray direct-drill: A more costly establishment technique where controlling pasture pests (e.g. slugs, porina, grass grub) and broadleaf weed challenges (post-establishment herbicide) are essential to ensuring high levels of success. Slug populations can build very quickly causing significant seedling loss. Where this is anticipated it is best to mix a molluscicide pellet (e.g. Metarex) with the seed (or via specialist drill box) at sowing. It is recommended that dressed, non-treated slug monitoring boards be deployed across the paddock straight after drilling. These should be checked every 2-3 days and where slug populations exceed 7-10/m2, the broadcasting of slug pellets will be needed.

Where insect pest challenge is expected a broad-spectrum insecticide can be added to the glyphosate at spray-out. Note: check the pH of the spray mix before application. Glyphosate’s greatest efficacy occurs within a pH range of 4.5-5.5. Where water sources are above pH 6.0 it is important to use an acidifier product (eg. penetrant product, like Li-700). This is particularly important when controlling mature, wild tall fescue.

As there is a greater impact on CCC with spray direct-drill paddocks out of the rotation longer, the proportion of the farm renovated annually will likely be lower (ie. 5-10% p.a.).

Double spray direct-drill: As above, but with a 3-4 week fallow where weed seeds are encouraged to germinate before a second spray is applied just before drilling. Works well where grass weed species are anticipated (eg. wild tall fescue, old ryegrass, Yorkshire fog) in the new seedbed, or where hay or baleage has been fed to stock over the winter (ie. a sacrifice paddock).

Spray cultivation: The most costly technique with the highest potential for successful establishment if executed well (well consolidated seedbed). Typically, less pest control is needed, but greater broadleaf weed challenge that has to be managed early (MCPB as soon as white clover seedlings have their third trifoliate leaf). Can impact heavily on CCC with only a 5-10% p.a. renewal rate being feasible. This can be offset to some extent by sowing a summer forage crop (eg. brassica, chicory) before autumn regrassing.

Pasture Species and Cultivar Selection

First, account for farm’s soil type, climate, topography and when feed is required by considering stocking rate, calving date and expected days in milk (DIM). Typically there are three component species of modern dairy pastures are all applicable to Bay of Plenty farming. These include: grasses (perennial and hybrid ryegrass types), legumes (white clover, red clover), and forage herbs (chicory, plantain). Other perennial grass species may include tall fescue and cocksfoot.

Ryegrasses

As a base pasture species, ryegrass is difficult to go past for its vigorous establishment, efficient solar radiation capture and flexibility in grazing management styles. Weaknesses can include: reduced persistence and dominance from establishment of important companion species (clover, forage herbs) that contribute much needed additional dietary energy (ME) and minerals.

Selection criteria for perennial and hybrid ryegrasses

Ploidy type (chromosome number): For some years most ryegrass cultivars have been released in the diploid format, with their higher tiller density (numbers) offering increased persistency potential. Increasingly there are a number of perennial, hybrid, Italian and annual ryegrass cultivars being released as tetraploids (ie. double the chromosomes). Advantages of these relative to diploids include: greater DOM and ME, higher VFI, reduced GHGs and increased animal performance (8-10%+). Weaknesses can include: reduced persistency (ie. selective grazing pressure), lower tiller density, and, depending on grazing management, increased dominance of the important companion species (white clover and forage herbs).

Endophyte type: It is now generally accepted that in order to gain both performance (yield) and persistence in our changing climate, a ryegrass cultivar benefits from having a bred stain of novel endophyte, almost irrespective of the NZ region it is used in. The more forgiving regions of Southland and the West Coast are likely to get good performance from the earlier novel endophytes (AR1 and NEA), while for many North Island regions the more modern types (AR37, NEA 2, NEA4, NEA12) are better suited as they offer wider pest species tolerance (e.g. black beetle, root aphid) and therefore increased persistency and yield.

Heading date (seedhead emergence): Most diploid perennial ryegrass cultivars have mid-heading dates (+0-8 days) with associated early spring growth, early seedhead emergence and persistent aftermath heading. Increasingly, more ‘late’ heading date (+12-20 days) cultivars have been released to the market with the aim of holding pasture quality later into the spring and reducing the post-peak lactation decline. There also a group of ‘very-late’ cultivars with heading dates of +21-25 days. Care needs to be taken not to have a farm dominance (ie. > 40%) of late-heading cultivars to avoid early-spring feed deficits. In pasture mixtures sow tetraploid cultivars at a 20-35% higher rate than diploids, as their seed weight is 2x heavier.

For the final selection from the myriad of ryegrass cultivars currently on the market, check how these three characteristics integrate and match the farm’s production objectives. NZ plant breeders have invested heavily to bring the farmer superior plant genetics to their older regressive pastures. Replacement rates of a minimum of 10% p.a. are needed to maximise the gains from these innovative and highly productive plants.

Companion pasture species

Clovers: An essential component of any NZ pasture. Advantages relative to ryegrass include: high forage quality (high levels of protein and soluble carbohydrate with MS responses 2.5x greater). Increased VFI. Greater mineral concentrations (macro and micronutrients). Ability to fix atmospheric N for utilisation by grass species (rates linked to DM yield – 25 kg N/t of DM). Increasing the proportion of white clover in our modern dairy pastures will reduce dependence on urea N keeping farmers well inside the current annual cap for synthetic N. Aim for a clover contribution of 25-40% of total pasture DM.

Forage herbs:

Chicory – is an energy dense, short-lived (1-4 years) perennial species that is highly palatable to all forms of livestock. Characterised by: strong growth from mid spring through to the late autumn. Chicory leaf has high levels of ME (12.5+) with low levels of structural fibre (high VFI). For persistence, chicory does require relatively free draining soils and rotational grazing. Can be used as an annual forage crop, typically sown in the spring for boosting cow energy intake throughout the lactation.

Plantain – A valuable companion species that has good energy and mineral contribution, that is more durable than chicory and provides good cool-season growth. Importantly, plantain also reduces the levels of soil nitrate N and therefore leaching. Note: where significant broadleaf weed challenge is anticipated then it pays to leave out both forage herb species of the sown mix and introduce these separately by oversowing following effective weed control.