In northern Ghana, harvesting of the bulk of paddy, which is cultivated under rain-fed conditions, coincides with the harmattan season. This season is characterized by low relative humidity and dry weather conditions. This condition hastens drying and increases the tendency of the paddy to crack during milling. In addition, smallholder farmers tend to store paddy for longer periods in anticipation of higher prices in the open market. Thus, the paddy may deteriorate during storage as a result of the seasonal fluctuation of temperature and relative humidity.

Rice cracking was known as sun-cracking by earlier researchers, latter research led to the observation that rice cracking was due to moisture absorption of dried rice (Schluterman and Siebenmorgen, 2007; Akowuah et al., 2012). Some studies in different varieties found that rice cracking begins at moisture content below 14.2 and 18.3% for crack resistant and crack susceptible varieties, respectively (Kumoro et al.,  2019). Another factor that may cause paddy to crack before harvest is nonsynchronous blooming which leads to uneven grain maturity (Schluterman and Siebenmorgen, 2007; Akowuah et al., 2012; Kumoro et al., 2019). In bulk storage of paddy, moisture migration causes low-moisture grains to absorb moisture from the high-moisture grains and crack when the high-moisture grains had only 4 to 5% more moisture than the low-moisture grains (Akowuah et al., 2012). Thus, grains can crack before harvesting, during threshing, in bulk storage, and during transportation. The study assessed the relationship between rice cracking and breakage during milling and the effect of storage duration on cracking. Furthermore, the effect of three fertilizer application regimes on grain filling and cracking was evaluated.

 

Statistical analysis

The data sets collected were subjected to analysis of variance, and treatment means separated using Least Significant Difference at 5% significance level. Correlation and regression relations between cracking and other parameters (% moisture content, duration of storage, days to milling, etc.) were computed.

 

Response of varieties to milling

Most rice varieties are composed of roughly 20% rice hull or husk, 11% bran layers, and 69% starchy endosperm, also referred to as the total milled rice. In an ideal milling process this will results into the following fractions: 20% husk, 8 to 12% bran depending on the milling degree and 68 to 72% milled rice or white rice depending on the variety. In this study, Amankwatia, recorded the highest head rice yield of 58.5% while Gbewaa recorded the lowest of 19.1%. Percentage broken was relatively low, ranging from 1.1 to 6.2% (Table 1). This may be due to the fact that the paddy was dried to the desired moisture content and milled immediately. Low percent cracking was observed in CSIR-AGRA (11.3%) and Amankwatia (12.9%) as compared to Gbewaa (13.6%) and Exbaika (14.7%). In general, the recommended moisture content to mill rice is 14%, at this moisture content, rice has the maximum mechanical strength. Above this  moisture,  the grain is soft and would crush during milling. At moisture contents less than 14%, the grain becomes brittle and cracks or break during milling. Both instances lead to reduction in head rice yield (Schluterman and Siebenmorgen, 2007; Akowuah et al., 2012; Kumoro et al., 2019). Generally, a good quality paddy constitutes about 55% head rice, 15% broken, 20% husk, and 10% bran. rice yield)

 

 

The results in Table 2 establish no significant relations between grain cracking and breaking during milling. Thus, all cracked grains will eventually break during milling. This may be due to the fact that some cracks may not go through the entire grain, while other cracks may be at the extreme tip of the grain. Although, there was no significant difference among the varieties in relation to percent crack and percent breakage, relatively higher percent breakages were observed in Exbaika (14.9%) and Gbewaa (16.5%) compared to CSIR-AGRA (13.2%) and Amankwatia (11.2%) (Figure 1). Figure 2 depicts the effect of variety on grain cracking across locations over a period of storage time. It can be observed that CSIR-AGRA (40.4%) and Exbaika (40.3%) showed relatively lower mean percentage cracking as compared to the other two varieties. Gbewaa rice recorded the highest percent cracking (49.2%).

 

 

 

 

Effect of length of storage on cracking

Figure 3 shows the effect of length of storage on percent cracking. Generally, percent cracking ranged  from  about 38 to 54%. Depending on the storage duration and prevailing ambient conditions, the quality of grain will remain same or deteriorate in storage. There was no significant difference in cracking across the varieties at 0 and 30 days after storage. However, by 60 days after storage, Gbewaa rice showed significantly higher cracking percentage compared to three other varieties.

 

 

Effect of fertilizer management on cracking

Use of appropriate nitrogen application regimes has been postulated to enhance hardness and reduce breakages in rice (Rehman et al., 2013; Faraji et al., 2013; Tari and Amiri, 2015). Current practice of two split applications of the recommended rate has been found to result in inefficient use of N. In this case, most of the N applied is not available at the grain filling stage when presumably; the plants require lots of nutrients to accelerate grain filling (Tari and Amiri, 2015). Such grains are thin, dry-out readily and susceptible to cracking. Finding adequate nitrogen management regime in rice production could result in the production of grains that are well filled, thick to resist drying and resistant to cracking.

Figure 4 shows the effect of fertilizer treatment on grain cracking after three months of storage in northern Ghana. There was no significant difference between the first two splits, FR1 (1 basal + 1 topdressing at maximum tiller stage) and FR2 (2 basal + 1 topdressings at panicle initiation) except in Exbaika, where there was a significant differences. However, significantly lower percent cracking was observed in all four varieties at FR3 (2 basal + 2 topdressings at panicle initiation and booting-heading). This indicates that all four varieties responded significantly to increased split fertilizer  rates.  The  higher the split fertilizer application, the more resistant the paddy is to cracking. These results correspond well with several earlier findings (Faraji et al., 2013; Rehman et al., 2013).

 

 

 

 

 

 

 

 

 

Cracks in paddy can occur before harvest, during threshing, drying, storage and even transportation depending on the ambient conditions. In this study, there was no direct relationship between paddy moisture content and percent cracking across the varieties. Overall, CSIR-AGRA Rice showed lower susceptibility to cracking compared to other 3 varieties. Fertilization regime of 2 basal applications and 2 topdressings at panicle initiation and booting-heading stages resulted in grains that were well-filled and thick to resist drying and cracking. Across the varieties, grains that showed signs of cracks will eventually break during milling, and grain susceptibility to cracking significantly increased when stored beyond 30 days.  It is therefore recommended that farmers and aggregators do not keep their paddy for too long as that could affect milling quality. Where storage is inevitable, the environment should minimize possible exposure of the paddy to absorb moisture.

 

The authors have not declared any conflict of interests.