3.2.1. Plant Growth and Forage Dry Matter

Average pearl millet plant population was 14.6 and 30.9 plants m−<sup>2</sup> in 2016 and 2017, respectively, a difference of 52%. A year x till interaction was observed, most likely due to low plant population in no-till in 2016 compared to no-till in 2017 (7.9 vs. 45.8 plants m<sup>−</sup>2, respectively). Additionally, in 2016, average population in no-till was 67% less than till (7.9 vs. 21.2 plants m−2, respectively). However, in 2017, average till plant population was 65% less than no-till (16.1 vs. 45.7 plants m<sup>−</sup>2). The no-till planting in 2016 was into herbicide killed native perennial grasses which prevented adequate seed to soil contact for proper germination. In 2017, an afternoon rainstorm following a morning irrigation event three days after sowing caused runoff that was observed in the till treatment. In contrast, no-till in 2017 was protected by the winter wheat stubble remaining on the soil surface after planting.

Forage DM was influenced by year, irrigation level, and tillage (*p* < 0.05) as main effects in both years; row spacing had no effect on forage production (Table 6). Average pearl millet DM in 2016 was approximately 38% less than in 2017 (1712 and 2759 kg DM ha<sup>−</sup>1, respectively). Average WU was 523 and 395 mm in 2017 and 2016, respectively. Average WUE was 5.28 and 4.26 kg ha−<sup>1</sup> mm−<sup>1</sup> in 2017 and 2016, respectively (Figure 2). Forage DM in H irrigation was 46% greater than L, ranging from 1219 to 2213 kg ha−<sup>1</sup> in 2016. The H level was 54% greater than L in 2017, which ranged from 2213 to 3450 kg ha−<sup>1</sup> (Figure 2). Dry matter in M was not different from H in either year but was different from L (*p* < 0.05; Figure 2). Water use in H was 15% greater than M and 43% greater than L in 2016. In 2017, H received 59% more water than L and 21% more than M (Figure 2). Water use efficiency was not different among irrigation levels ranging from 3.78 to 4.79 kg ha−<sup>1</sup> mm−1. In contrast, a C3 bioenergy crop, hybrid poplar (*Populus generosa* Henry × *P. nigra* L.), achieved differing WUE values of 5.6 and 8.9 kg ha−<sup>1</sup> mm−<sup>1</sup> grown under 115 mm and 240 mm of applied irrigation, respectively [35]. Row spacing did not influence pearl millet forage DM, WU, or WUE in 2016 or 2017 (Figure 2, Table 6).


**Table 6.** The *p*-values from ANOVA of pearl millet dry matter (DM), water use (WU), and water use efficiency (WUE).

<sup>a</sup> I, irrigation; R, row spacing; T, tillage; ns, not significant at *p* = 0.05.

Pearl millet DM and WUE in the tilled plots were greater than in no-till in both years, although, WU was not affected by tillage management in either year (Figure 2). Forage DM in till was 66 and 39% greater than no-till, in 2016 and 2017, respectively. Pearl millet WUE in tilled soil was 68% greater than no-till in 2016 and 39% greater in 2017 (Figure 2).

Forage DM in 2016 till was greater as a consequence of the low plant population in no-till due to poor seed to soil contact. The low forage yields in 2017 are attributed to N deficiency observed in the no-till plants. This deficiency in no-till may be a result of higher plant population, compared to the till treatment, leading to the deficiency observed mid-season and no deficiency observed in the till plants.

Crop canopy height in 2016 was not influenced by irrigation, row spacing, or tillage; the end of season canopy height was 50 cm. For the 2017 season, canopy height was affected by irrigation over time. Height at harvest was 72, 67, and 59 cm for H, M, and L, respectively. In addition, there was an irrigation × tillage interaction in 2017, where L × no-till was 38 cm and H × till was 49 cm at harvest. No difference for crop canopy height was measured in the tillage treatment for either year.

**Figure 2.** (**A**) pearl millet dry matter; (**B**) water use = (rainfall + irrigation + plant available soil moisture at planting)–plant available soil moisture at harvest; (**C**) water use efficiency = dry matter/water use. Lowercase letters indicate significance at *p* < 0.05 within treatment and year; bars indicate standard error within treatment. H, high irrigation level, M, moderate irrigation level, L, limited irrigation level; 19 cm and 76 cm row spacing; N-t, no-till, T, tilled.
