Discipline: nutritive value; Keywords: maize hardness, near-infrared spectroscopy, PSI, RVA, intra-lab analysis, feed industry.

Maize is the most important non-forage carbohydrate source in high yielding dairy cow diets. Both ruminal and total tract starch digestibility of dairy and other ruminant animals are significantly impaired by high vitreous maize, compared to moderate floury or dent maize. The genetic make-up, environmental conditions of cultivation and stage of maturity collectively affect the hardness of maize. Various methods, including particle sieve (106 μm) index (PSI), near-infrared spectroscopy (NIR), rapid visco analyser (RVA) and x-ray micro-computed tomography (XCT), are currently used to determine maize hardness. The aim of the study by the authors cited below was to evaluate NIR technology and compare it with other methods of maize hardness determination in order to establish if the rapid NIR method is a useful and sufficiently accurate method of determining maize hardness in the ruminant feed industry.  

Ninety maize samples, mainly from South Africa, but also including a few from Argentina and Ukraine, were collected to obtain a diverse set in terms of hardness. The hardness of all 90 samples was determined in Trial 1 by PSI, using a single 106 μm screen, and NIR at a single absorbance of 2230 nm. The hardness was tested in relation to origin, colour (yellow versus white) and cultivation method (irrigated versus rain fed). Based on the results of Trial 1, ten hard and ten soft samples were selected for Trial 2. These samples were used to evaluate the accuracy of maize hardness determination by means of three techniques, namely PSI, NIR and RVA. The XCT analysis was used as a reference.

The results showed that maize colour and cultivation did not affect hardness. Climatic conditions of origin showed significant differences between humid subtropical and cold semi-arid production areas. In Trial 1, results indicated a significant correlation (r2 = 0.74; P≤0.01) between PSI (106 μm sieve) and NIR (absorbance at 2230 nm). Accurate hardness determination by both methods was established using specific intra-lab analysis. In Trial 2, it was concluded that all the methods (PSI, RVA, NIR and XCT) could be equally effective to determine maize hardness. Whereas both PSI and NIR are practical, accurate, rapid and cost effective methods to determine maize hardness in the animal feed industry, XCT, RVA peak time and RVA peak viscosity did not satisfy all requirements. 

As NIR technology is already available and utilised extensively in the animal feed industry, it was concluded that NIR at a single absorbance of 2230 nm meets all the requirements  to determine maize hardness.


J.H.C. van Zyl & C.W. Cruywagen, 2019. Evaluation of methods to determine maize quality and hardness (vitreousness). In: Proc. of the 51st Annual Congress of the SASAS, Bloemfontein, 10-12 June 2019, Abstract 206.