3.6.1. De-trended Maize Yield Correlation with rainfall, Tmin and Tmax Anomalies

The Pearson correlation coefficient (*r*) and confidence interval levels of 0.05, 0.01 and 0.001 were used in this study to determine the relationship between yield and agroclimatic variables. Rainfall was positively correlated with yield during the growing period in Clocolan and with Tmax in Senekal (*r* = 0.46 and 0.48 respectively) (*p* = 0.008 and 0.0005 respectively) (Table 9). In November, only the Tmin in Marquard correlated with yield (*r* = 0.39, *p* < 0.027). During the month of January, the yield at this station is positively correlated with Tmin (*r* = 0.37 and *p* = 0.038 at 0.05 confidence level).




The minimum temperatures were correlated with maize yield only for the Marquard station in the months of November and February, this relationship was also found to be the case in studies conducted by Adisa, Botai [70]. Temperature drives the physiological and morphological development of the maize plant, with each process requiring a different minimum and maximum temperature. For instance, the study by Sanchez, Rasmussen [71] showed that leaf initiation needs a minimum of 7 ◦C, while shoot growth takes place above 14 ◦C and root growth above 13 ◦C. These minimum temperature conditions were not met for all cases except for the leaf initiation process in November (Table 5 above). However, in January the minimum temperature requirements for leaf initiation and shoot and root growth were met even for the late planting cultivars. Minimum temperatures, especially in November, seem to be critical for the early establishment and growth of the seedlings which ultimately influences the yield. The correlation and the regression analyses provided evidence for the significance of the minimum temperature on yield in Marquard, especially in the months of November and January. However, the November minimum temperature trend showed an increase of 0.09 ◦C per annum (see Table 5 above), which showed an increase of 1% in Tmin in November increasing the yield by 0.274 tons ha−<sup>1</sup> in Marquard. Climate change predictions for semi-arid regions of SSA have changed from earlier studies which gave values of 1.6 ◦C to recent projections of above 2.4 ◦C by 2050, depending on emissions and other anthropogenic activities [72]. Increasing trends in minimum temperatures are predicted for SSA, and extreme climate events, especially the frequency and severity could negatively impact yields [73].

The February Tmax was negatively correlated with yield in Marquard and positively in Senekal (*r* = −0.49 and 0.657; *p* = 0.005 and <0.001 and 835.835, respectively) at 0.01 and 0.001 confidence levels, respectively. Similarly, the February rainfall in Marquard was positively correlated with yield (*r* = 0.42, *p* = 0.018) at 0.05 confidence level. There was also a strong correlation between them in Clocolan (*r* = 0.69, *p* < 0.001 and) in the month of February, while in March, the Tmax in Senekal showed a positive correlation (r = 0.4512 *p* = 0.003) at 0.01 confidence level with yield (Table 9).

The results from this study showed that the maximum temperatures for the entire growing season were significantly correlated with maize yield only for Senekal. This was as a result of the significant correlation in the months of February and March. The stations of Clocolan and Ficksburg showed no correlation between the Tmax and maize yield, while those in Marquard showed a significant negative correlation. The results in Marquard were also similar to other studies which showed that temperatures above 30 ◦C have a negative impact on maize production in southern Africa [74]. Senekal had the lowest maximum temperatures and a 1% increase of Tmax in the months of February, March and the entire growing period (October–April) could increase the maize yield by 0.029, 0.408 and 0.536 tons ha−<sup>1</sup> (Table 9). On the other hand, Marquard had the highest maximum temperatures and a 1% increase of Tmax could decrease maize yield by 0.290 tons ha<sup>−</sup>1. Lobell, Bänziger [74] showed that a 1% increase of maximum temperature above the optimal temperature for growth under drought stress could result in a maize yield decline of 1.7%. Clocolan had the highest mean Tmax value and SD value of 28.6 ◦C and 2.4 ◦C respectively. There are several other studies that showed that high temperatures, together with soil and plant water stress lead to a decline in crop yield [75,76]. Maize yield in Marquard will be most vulnerable to water stress if the maximum temperatures continue to increase, especially at the anthesis stage, where the optimal temperature is 32 ◦C and the maximum tolerable Tmax is 36 ◦C [58]. Muchow (1990) showed that temperatures outside the range of 13–32 ◦C decrease the yield by shortening the period of the kernel filling. These conditions also apply in Marquard with high February maximum temperatures which prevailed when kernel filling would have taken place if planting took place in November.

3.6.2. Maize Yield Relationship with Rainfall, Minimum and Maximum Temperature Anomalies

The monthly minimum, and maximum temperatures, as well as the rainfall that showed a significant correlation with maize yield (see Table 9 above) were subjected to regression analysis. The yield was the dependent variable while monthly Tmin, Tmax and rainfall were the independent variables used across the different stations of the Setsoto Municipality. The influence of the Tmin on

maize yield during the months of November and January in Marquard were significant (*p* < 0.00027 and *p* < 0.038, respectively) (Table 10). The Tmax during the month of February showed a significant negative impact on maize yield when regression analysis was conducted (*p* < 0.005, R<sup>2</sup> = 0.23) whilst for the same month, rainfall showed a positive impact on the maize yield in Marquard. An increase of one unit of rainfall in (mm) can increase the yield by 0.0921 tons ha−<sup>1</sup> (Table 10).


**Table 10.** A summary of regression results between detrended maize yield and the climatic (Tmin, Tmax and Rainfall) anomalies. Note: *p* = *p*-value at 0.05.

In Senekal, maximum temperatures in the months of February, March as well as the entire growing period (October–April) had a significantly positive impact on the maize yield (*p* < 0.05) (Table 10). In February, for every increase in degree Celsius of Tmax above the base temperature led to an increase of the yield by 0.3459 tons ha−<sup>1</sup> year−1, while an increase in Tmax in March and the whole season of the growing period (October–April) led to an increase of maize yield by 0.367 and 0.592 tons ha−<sup>1</sup> respectively in Senekal (Table 10).

The effect of rainfall during the growing period and the month of February in Clocolan, showed a significant and positive relationship with the maize yield (*p* < 0.05) (R<sup>2</sup> = 0.214 and 0.472, respectively). An increase in rainfall by a unit (mm), increased the yield from 0.1028 to 0.1179 tons ha−<sup>1</sup> year−<sup>1</sup> (Table 10).
