No abstract

Economic values (EVs) are an estimate of the change in farm system profit per unit of change in a defined plant trait. Cultivars within species such as perennial ryegrass differ in the major production traits of dry matter yield, nutritive value and persistence, but the impact of those differences on farm financial performance is seldom calculated and reported. This paper explains what EVs are, describes how they can be calculated, and discusses some of the associated methodological issues. EVs have been derived for seasonal dry matter yield in New Zealand dairy systems. Extra feed produced in late spring has consistently low economic value, while extra feed produced in early spring has consistently high value. There have been no systematic investigations into the EVs of nutritive value and persistence in New Zealand pastures; this is a clear opportunity for future research. The lack of data on cultivar differences in these traits will restrict the application of EVs to pasture cultivar evaluation. This gap is now being addressed by new industry initiatives. Keywords: pasture cultivars, evaluation, traits, economic values, performance values

The largest contributor to nitrogen (N) leaching from ryegrass-clover pasture based dairy farms is the surplus feed N excreted as urinary N (UN) onto pastures. Pastures consisting of mixtures of ryegrass, herbs and legumes (diverse pastures) have shown potential to yield similar DM, but with a lower N content and a higher water soluble carbohydrate : crude protein ratio compared with standard ryegrass–clover pastures. These diverse pastures have shown the potential to lower the UN excreted by dairy cows in short-term, late-lactation studies. This modelling study was designed to scale the results from component studies up to farm and over a full season to evaluate the potential of diverse pastures to become a suitable strategy for reducing N leaching on New Zealand dairy farms. The Molly cow model was tested against observed data from one indoor and one outdoor study where feeding diverse pasture resulted in UN (N excreted in urine g/day) reductions of 50% and 17%, respectively. The model predicted UN reductions of 23% and 17%. Farm-scale model scenarios, where 20% or 50% of the farm was sown with diverse pastures, resulted in 2% and 6% reductions in UN deposited onto paddocks. This reduction was smaller than expected with some system interactions related to seasonal feed supply, diet composition and total N intake being likely to play a role. The reduction in UN onto paddocks, together with a dilution effect from larger urine volumes per cow per day as a result of lower DM% of diverse pastures, resulted in N leaching reductions of 11% and 19% for the two scenarios, respectively. This potential to reduce N leaching needs to be evaluated further in the context of farm profitability when other aspects of diverse pastures such as yield, persistency, drought resistance and ability to extract N from the soil becomes part of the farm-system analysis.

This study tested the hypotheses that: (i) genetic variation in Rubisco turnover may exist in perennial ryegrass (Lolium perenne L.); (ii) such variation might affect nitrogen use efficiency and plant yield; and (iii) genetic control of Rubisco turnover might be amenable to identification by quantitative trait loci (QTL) mapping. A set of 135 full-sib F1 perennial ryegrass plants derived from a pair cross between genotypes from the cultivars 'Grasslands Impact' and 'Grasslands Samson' was studied to test these hypotheses. Leaf Rubisco concentration at different leaf ages was measured and modelled as a log-normal curve described by three mathematical parameters: D (peak Rubisco concentration), G (time of D), and F (curve standard deviation). Herbage dry matter (DM) yield and morphological traits (tiller weight (TW), tiller number (TN), leaf lamina length (LL), and an index of competitive ability (PI)) were also measured. The progeny exhibited continuous variation for all traits. Simple correlation and principal component analyses indicated that plant productivity was associated with peak Rubisco concentration and not Rubisco turnover. Lower DM was associated with higher leaf Rubisco concentration indicating that Rubisco turnover effects on plant productivity may relate to energy cost of Rubisco synthesis rather than photosynthetic capacity. QTL detection by a multiple QTL model identified seven significant QTL for Rubisco turnover and nine QTL for DM and morphological traits. An indication of the genetic interdependence of DM and the measures of Rubisco turnover was the support interval overlap involving QTL for D and QTL for TN on linkage group 5 in a cluster involving QTL for DM and PI. In this region, alleles associated with increased TN, DM, and PI were associated with decreased D, indicating that this region may regulate Rubisco concentration and plant productivity via increased tillering. A second cluster involving QTL for LL, TN, PI and DM was found on linkage group 2. The two clusters represent marker-trait associations that might be useful for marker-assisted plant breeding applications. In silico comparative analysis indicated conservation of the genetic loci controlling Rubisco concentration in perennial ryegrass and rice.

Despite the importance of roots in determining plant performance, the factors controlling their development and longevity remain poorly understood. Grass morphology is based on repeating units called phytomers, with each capable of producing one leaf, one daughter tiller, and one or more roots. We developed a phytomer-based understanding of root birth, growth and senescence in Lolium perenne, using a modeling approach to explore seasonal effects on root turnover dynamics, and to explore cultivar differences in these processes. Similar to leaves, roots exhibit a clear progression from initiation, growing for approximately seven phyllochrons, with growth rates strongly influenced by environmental conditions. In spring, the phyllochron decreased over the experiment, while it increased in autumn. In spring, C availability exceeding maintenance respiratory requirements allowed root growth at each phytomer position, with a 70/30 split between maintenance and growth. Under C-deficient conditions in autumn, this split was approximately 80/20, with growth limited to younger phytomer positions, while older roots were more susceptible to starvation-induced senescence due to their high C requirements for maintenance respiration.