Damage to roots of Trifolium subterraneum L. (subterranean clover), failure of seedlings to establish and the presence of root pathogens during autumn–winter

Field‐based plant bioassays were employed to assess the potential for pre‐ and post‐emergence loss of seedlings and for root damage affecting Trifolium subterraneum L. (subterranean clover) during autumn–winter at 17 pasture sites across a broad agricultural area of temperate southern Australia. Between 9 and 93% (median 21%) of T. subterranean seedlings failed to emerge at the 14 locations where soil moisture was considered adequate for germination. Post‐emergence losses were lower (range 0–32%; median 7%). Moderate damage (lateral roots) to severe damage (taproots) was recorded on surviving test plants at all of the sites. Sublethal damage to pasture roots constitutes a potentially large, but underestimated cost to production because it was so widespread and because the damage occurs during autumn–winter when pasture yield limits stocking rate. Potential for the loss of Lolium rigidum × multiflorum (annual ryegrass) seedlings was also demonstrated at some of the sites. DNA assays for common root rot disease pathogens (Pythium irregulare, Phytopthora clandestina and Rhizoctonia) were used for the first time to construct cost‐effective profiles of fungal and oomycete pathogens at each site. These assays may be useful for indicating disease risks and guiding plant cultivar selection and appropriate use of pesticides. DNA assays for arbuscular mycorrhizal fungi were also used and have the potential to provide novel insights into the soil biology of farming systems.     

Independence of seasonal patterns of root functional traits and rooting strategy of a grass‐clover sward from sward age and slurry application

Root functional traits (RFTs) are crucial for understanding nutrient cycling processes. However, seasonal variation of RFTs and their potential changes in root foraging pattern have been almost ignored in field studies. In this work, belowground net primary production (BNPP, g m^?2), root length density (RLD, km m^?2), specific root length (SRL, m g^?1), root N concentration and C/N ratio in defoliated grass‐clover swards of different sward ages and receiving different cattle slurry levels were investigated for 2 years. All measured RFTs showed significant seasonal variation: in general, seasonal variation was most marked between primary growth period (until heading stage of perennial ryegrass) and subsequent regrowth periods following defoliation; meanwhile, the rooting pattern also differed. These phenomena were not affected by current management or sward composition, but were correlated with plant C availability. Daily BNPP and RLD rates varied 7‐ to 8‐fold in the growing season. Moreover, BNPP and RLD were asynchronous due to seasonal variation in SRL. Rooting pattern and the relationship with shoot phenology are discussed. We conclude that root functional traits vary with season; thus, roots which develop in different seasons may be involved in belowground nutrient cycling processes with different turnover characteristics; trade‐off among traits may also involve temporal variation. Root‐C related investigations in grass‐clover swards should include root mass sampling rather than use of root length observations alone.     

Effect of grazing management on herbage protein concentration,milk production and nitrogen excretion of dairy cows in mid‐lactation

The objective of this experiment was to use diurnal and temporal changes in herbage composition to create two pasture diets with contrasting ratios of water‐soluble carbohydrate (WSC) and crude protein (CP) and compare milk production and nitrogen‐use efficiency (NUE) of dairy cows. A grazing experiment using thirty‐six mid‐lactation Friesian x Jersey cows was conducted in late spring in Canterbury, New Zealand. Cows were offered mixed perennial ryegrass and white clover pastures either in the morning after a short 19‐day regrowth interval (SR AM) or in the afternoon after a long 35‐day regrowth interval (LR PM). Pasture treatments resulted in lower pasture mass and greater herbage CP concentration (187 vs. 171 g kg^?1 DM) in the SR AM compared with the LR PM but did not affect WSC (169 g kg^?1 DM) or the ratio of WSC/CP (1·0 g g^?1). Cows had similar apparent DM (17·5 kg DM cow^?1 d^?1) and N (501 g N cow^?1 d^?1) intake for both treatments. Compared with SR AM cows, LR PM cows had lower milk (18·5 vs. 21·2 kg cow^?1 d^?1), milk protein (0·69 vs. 0·81 kg cow^?1 d^?1) and milk solids (1·72 and 1·89 kg cow^?1 d^?1) yield. Urinary N concentration was increased in SR AM, but estimated N excretion and NUE for milk were similar for both treatments. Further studies are required to determine the effect of feeding times on diurnal variation in urine volume and N concentration under grazing to predict urination events with highest leaching risk.