Comparison of Sowing Rate and Seed Treatment on Kikuyu Pasture Dry Matter Yields

Trial plots were established on three dairy farms in Northland in late April 2009 and were monitored from May to December for differences in total pasture dry matter (DM) yield after direct-drilling Italian ryegrass into kikuyu (Pennisitum clandestinum) pasture following mulching. Pesticide treated or untreated Italian ryegrass (Lolium multiflorum) was drilled at 10, 15, 20 and 30 kg seed per hectare. Each sowing rate and seed treatment was replicated twice on each of three farms. Plot dry matter (DM) yields were estimated approximately monthly preand post-grazing using a rising plate meter, and thirteen growth periods were measured in total over the three sites. Daily growth rates and total DM yields varied between sites and were not consistent at any of the sowing rates. While significant treatment differences in yield were only apparent at one of the sites there was a trend for total yield to increase with increasing sowing rate. However, the yield response at 30 kg seed per hectare was similar to the response from 20 kg seed/ha. There was no significant pasture yield response to pesticide treatment of seed. This work suggests that sowing Italian ryegrass into kikuyu at 20 kg per hectare is beneficial to total pasture yield and is in line with seed merchant recommendations. Introduction Northland pastures are typically made up of perennial ryegrass (Lolium perenne)/clover (Trifolium repens) swards with varying amounts of C4 grasses such as kikuyu (Pennisitum clandestinum) and paspalum (Paspalum dilatatum) interspersed. Up to 70% of Northland farms (or 5.5% of national milk production) are affected to some degree by C4 grasses and their management continues to be a high priority for farmers (Wood and Andrewes, 1990). Soil temperatures permit ryegrass growth during winter, but total pasture growth rate is dependent on the proportion of C4 grasses present in the sward. While more frost tolerant than many warm season grasses, exposed kikuyu foliage will be damaged by heavy frosts (<-2oC). A trial carried out by MAF in the early 1970s showed that from April to August monthly kikuyu pasture production was strongly correlated with temperature variables and for every one degree decline in daily grass minimum temperature, pasture production decreased by 3.4 kg DM/ha/day (KAG, 2007). Drilling annual or Italian ryegrasses into kikuyu pastures after autumn mulching has been shown to increase winter and early spring pasture production, but much of the available information is anecdotal. Kikuyu grows horizontally mainly via stolons. Mulching is the process of breaking up the stolons of the kikuyu plant by using individual blades mounted on a horizontally rotating drum to swing and cut vertically compared to the horizontal motion of mower blades. Mulching when kikuyu growth is slowing down due to cooler soil Comparison of Sowing Rates and Treated Seed on pasture dry matter yields in Kikuyu pastures temperatures in autumn weakens kikuyu’s competitive advantage and wintering ability, leaving an opportunity for ryegrass production. A recent FITT funded study showed that undersowing of Tabu Italian ryegrass at 18 kg/ha into mulched kikuyu, increased DM production by 1560 kg /ha over the first nine months that it was monitored (Montgomerie and Boom, 2010). Increased feed supply during early spring is very important, as this period coincides with calving on the majority of dairy and beef cattle farms in Northland. While there are other options for farmers to feed stock during periods of slow winter growth, such as supplementary feed, nitrogen or grazing off, the best option for Northland farmers is to produce more pasture off their existing land. Seed companies supply information promoting the drilling of Italian ryegrass into C4 pastures in autumn using 20-25 kg seed per hectare. This represents a considerable cost of supplementary feed. However, the most economic usage of these grasses in terms of the sowing rate or seed treatment and effect on subsequent pasture dry matter (DM) yield remains unclear. The purpose of this experiment was to determine the optimal sowing rate for maximising Italian ryegrass growth with the aim of increasing DM production during periods of slow winter growth. Treated seed aids plant establishment by protecting seedlings from insects (springtails, Argentine stem weevil and aphids) and fungal diseases that contribute to establishment failure, and by supplying rhizobia and nutrients to clovers. Seed treatment was included as a variable to determine if there were any differences in establishment and growth rates due to seed treatment as this also increases the seed purchase price by 15 to 20%. Materials and Methods Experimental site, design and treatments The experiment commenced in May 2009 at three sites around Northland (Whangarei Heads, Dargaville and Kaitaia) on commercial dairy farms. Pasture DM production was estimated over seven months from June to December using a rising plate meter (RPM) pre-and post-grazing (variable times at all three sites depending on pasture growth and individual farm grazing rotations). The winter formula was used in all RPM equations (clicks x 140 + 500) to determine DM yield. Each treatment was replicated twice on each farm in 3 x 4 m plots arranged in a randomised block design (18 plots per farm) with 2 replicates. Treatments were four sowing rates of Italian ryegrass/clover (10, 15, 20 and 30 kg/ha) either treated (coated with pesticide) or untreated seed (16 plots) and a control that received no Italian ryegrass seed (2 plots) (Table 2). Insect pressure was not measured at any of the sites. Establishment and Management The plots, predominately kikuyu and white clover, were mulched to ground level in late April. They were then direct-drilled with the Italian ryegrass seed and white clover mix at the specified sowing rates. The seed was weighed for individual plots before drilling. The timetable of experimental events is shown in Table 1. Comparison of Sowing Rates and Treated Seed on pasture dry matter yields in Kikuyu pastures Table 1. Timetable of experimental events from April to December 2009 Event Date Plots mulched to ground level and drilled 22-28 April First pre-grazing measurements taken 15 June Preand post-grazing measurements taken June to December Final pre-grazing measurement taken 3 December Table 2. Plot treatment groups used at Whangarei, Dargaville and Kaitaia Seed Type Sowing Rate of Italian ryegrass/ clover seed (kg/ha) Control 0