Meeting the challenge of feeding a growing population with limited resources will require increasing the yield potential of staple crops, such as rice. Yet many high-yielding, intensive production systems have experienced slow rates of yield improvement in recent years despite a demonstrated increase in the yield potential of new crop cultivars. We analyzed experimental data from one such cropping system, i.e., California (CA) rice, in order to quantify improvements made in the genetic yield potential obtained through plant breeding. California rice systems are among the highest in the world and close to maximum yield potential. Specifically, the hypothesis was tested that if rice cultivar yields decline over time then apparent yield increases in side-by-side yield comparison tests will not reflect increases in yield potential. This hypothesis was tested using 33 years of experimental yield data from the California Cooperative Rice Research Foundation Rice Experiment Station. Based on side-by-side comparisons of old and new rice cultivars which do not consider yield decline over time, there was an apparent increase in yield. However, the yields of older cultivars were found to decline at an estimated rate of 29.3 kg ha−۱ year−۱ (۹۰% credible interval −۴٫۴ to −۵۳٫۳) after initial selection. Once this effect was considered, the yield advantage of newer cultivars over old was uncertain (−۳٫۳ kg ha−۱ year−۱ , ۹۰% credible interval −۳۶٫۱ to 31.5). These results highlight (1) the importance of continuous crop improvement and deployment of new cultivars simply to maintain existing yields, and (2) to increase the genetic yield potential, higher yield targets are needed. Importantly, when breeding near the yield potential, despite the limited yield gains, significant advances in improving quality and reducing crop duration have been made.

Introduction

Constraints on arable land are increasing simultaneous with the need to increase total food production to meet a growing demand (Foley et al., 2011; Godfray et al., 2010; Mueller et al., 2012), which has necessitated harvesting more grain per unit land area (Lobell et al., 2009; Tittonell, 2014). Historically, agricultural research has been successful in staving off the “Malthusian catastrophe” of demand surpassing supply via continued yield improvement of staple crops. However, many production systems are experiencing plateaus in grain yield (Grassini et al., 2013). If this trend continues, improvements per unit land area are no longer possible, and an increase in the area under cultivation will be needed to meet food demand, which carries undesirable ecological implications (Foley et al., 2011; Tilman et al., 2011). Therefore, it is of critical importance to better understand why the rate of increase in grain yields has declined or leveled off in intensified production systems. California (CA) rice represents one such production system. Rice is grown primarily in the Sacramento Valley, which is characterized by having a Mediterranean climate with long days, a dry growing season relatively free of pests and diseases. These conditions lead to some of the highest yields in the world FAOSTAT, 2016. Production in CA is predominately focused on premium quality medium grain japonica cultivars (e.g., CalRose rice), and rice from CA is recognized globally for its quality (http://agfax.com/2015/11/03/rice-calrose-wins-best-riceworld-competition/). Most cultivars in use in CA are developed by the California Cooperative Rice Research Foundation (CCRRF; a collaboration between the University of California, the USDA-Agricultural Research Service, and farmer-funded research). In part due to improvements in rice genetics, rice yields in CA increased rapidly during the period from 1920 to 1990; however, since the 1990s, the rate of yield increase has slowed (Fig. S2) despite continuous crop improvement. Based on a yield-gap analysis, on-farm yields in the major rice growing region of CA is 73–۷۶% of maximum yield potential (Espe et al., 2016). In highly intensive systems such as this, Grassini et al. (2011) have shown that farmers are capable of attaining 85% of the maximum yield potential. Thus, given that farmers are near the attainable yield potential, increases in yield are likely to be relatively slow and Espe et al. (2016) reported that indeed this was the case with yields on average increasing by about 50–۶۲ kg ha−۱ year−۱ between 1999 and 2014. Certainly one question in looking at this situation is, “do new cultivars have greater yield potential”?