*3.3. Irrigation Water Use Efficiency*

The average value of irrigation water use efficiency (IWUE) for the harvest years and cultivars was 31 kg·ha−1·mm−<sup>1</sup> (Table 13). Irrigation water use efficiency in the first year of research (2007) was lower (19 kg·ha−1·mm−1) than in the second year of study (2008) (44 kg·ha−1·mm<sup>−</sup>1). This was due to the fact that, for the irrigation season in 2006, a higher seasonal irrigation norm was used than in 2007. Obviously, asparagus yields harvested at drip irrigated plots in 2007 were the result of irrigation carried out in 2006, and, respectively, postharvest irrigation carried out in 2007 had an impact on asparagus yields harvested in 2008. Jersey Deluxe and Jersey Giant were characterized by the highest IWUE, which, on average, for 2 years of research, was 64 kg·ha−1·mm−<sup>1</sup> and 57 kg·ha−1·mm<sup>−</sup>1, respectively. The lowest IWUE (4 kg·ha−1·mm−1–6 kg·ha−1·mm−1) was found in Atlas, Jersey King, and Purple Passion cultivars. The other tested cultivars, on average, for 2 years of research, were described by IWUE as ranging from 26 kg·ha−1·mm−<sup>1</sup> to 41 kg·ha−1·mm<sup>−</sup>1. Diverse irrigation water use efficiency in the cultivation of various asparagus cultivars has been confirmed by the results of previous trials conducted in the central Poland (region of Bydgoszcz) with other cultivars of asparagus [2,14] and vegetable species [50,51] or berry plants [52–55].


**Table 13.** Irrigation water use efficiency (kg·ha−1·mm<sup>−</sup>1) as dependent on cultivar and year studied.

#### **4. Conclusions**

Drip irrigation of 13 cultivars of asparagus, applied for 2 years in the postharvest period (2006–2007), had a positive effect on all studied characteristics in both summer stalks and consumption green spears of this vegetable. On average, in 2006–2007, a significant increase in the height, number, and diameter of summer stalks was visible in 13, 12, and 9 of 13 studied asparagus cultivars, respectively. On average, in 2007–2008, the increase in marketable yield, weight, and number of green spears was significant for 12, 11, and 12 of 13 tested asparagus cultivars, respectively. The highest marketable yield increase of green spears under drip irrigation—above 7 t·ha−1, on average, for 2 years of research—was obtained for Jersey Deluxe, Grande, Apollo, and NJ 953 cultivars. The best response to drip irrigation was noted in Jersey Deluxe and Jersey Giant. Drip irrigation increased the marketable yields of these cultivars by 157% and 142%, respectively. Jersey Deluxe and Jersey Giant cultivars were characterized by the highest irrigation water use efficiency (calculated for the marketable yield of green spears), which, on average, for 2 years of research, was 64 kg·ha−1·mm−<sup>1</sup> and 57 kg·ha−1·mm<sup>−</sup>1, respectively. High irrigation water use efficiency, over 30 kg·ha−1·mm−1, was observed in the case of 9 of 13 tested cultivars. In summary, postharvest irrigation of asparagus cultivated on a very light sandy soil significantly improved both the summer stalk characteristics, as well as the marketable yield features of consumption green spears of this vegetable species. Drip irrigation of asparagus, applied after the harvest in the growing season preceding the next green spears harvest, makes it possible to significantly increase the productivity of this vegetable, which contributes to the sustainable crop production.

**Author Contributions:** Conceptualization—R.R., S.R., and A.F.; methodology—R.R., S.R., and A.F.; software—R.R., S.R., and A.F.; validation—R.R., P.S., and F.P.-F.; formal analysis—R.R., P.S., D.L., and F.P.-F.; investigation—R.R., S.R., and A.F.; resources—R.R. and S.R.; data curation—R.R. and S.R.; writing—original draft preparation—R.R., S.R., A.F., B.J., P.S., W.P., P.P., M.J.K., and H.A.S.; writing—review and editing—R.R., S.R., A.F., B.J., P.S., W.P., P.P., M.S., and M.J.K.; visualization—R.R., S.R., B.J., P.P., F.P.-F., M.J.K., and H.A.S.; supervision—R.R., S.R., B.J., P.P., F.P.-F., M.S., and M.J.K. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.
