Nutritional interest and quality
The quality of rice straw depends on many factors: variety, time between harvest and storage, N fertilization, plant maturity (lignin content increases with maturity), plant health and weather conditions (Göhl, 1982; Drake et al., 2002). Rice straw is a good source of energy, but is low in protein (2-7%) and its high silica content results in a low digestibility (Drake et al., 2002). It is considered as a low quality and variable roughage. Minerals (particularly sulphur) can be limiting factors (Doyle et al., 1986). Other limitations include:
- high NDF content resulting in poor DM intakes and low fat-corrected milk yields (Kanjanapruthipong et al., 2006);
- low concentrations of P, Cu, Zn, Ca and NaCl that do not meet animal requirements (Gowda et al., 2005);
- low energy content compared to maize silage and lower palatability (Odai et al., 2002), resulting in poorer N utilization.
When straws are fed to ruminants, the primary limitations to production are low overall digestibility, slow rate of passage in the rumen, low propionate fermentation pattern in the rumen, and low contents of fermentable N and by-pass protein.
Depending on the farming systems, there are several ways to feed ruminants with rice straw (Doyle et al., 1986):
- In extensive systems, the animals are allowed to enter paddy fields after harvest and graze rice straw and weeds in the field or on the roadsides.
- In less extensive systems, the animals are tethered in the paddy field close to stacks of rice straw.
- In stall-fed systems, rice straw may be fed alone, or fed with other forage supplements and/or concentrates.
Supplementation is strongly recommended in order to mitigate the nutritional weaknesses of rice straw.
In ruminants, when milk or meat production is desired, rice straw must be supplemented with both protein and energy sources. For good growth from straw-based diets, a level of 8 to 10% CP is needed for young stock: this also improves consumption and thus increases energy intake (Jackson, 1979).
Legumes are often suggested as supplementary protein sources. Many potential legume supplements have been reviewed and proved to give good results: vetch (Lathyrus sativus) (Akbar et al., 2000), stylo (Stylosanthes guianensis) (Khuc Thi Hue et al., 2008), griffonia (Griffonia simplicifolia) (Oddoye et al., 2005), gliricidia (Gliricidia sepium) (Kusmartono, 2007; Orden et al., 2000), leucaena (Leucaena leucocephala) (Orden et al., 2000), velvet bean (Mucuna pruriens) (Adjorlolo et al., 2001) and tithonia (Tithonia diversifolia) (Premaratne et al., 1998).
Other plants may also be used as protein supplements in a rice straw-based diet: cassava leaves or cassava hay (Vongsamphanh et al., 2004; Premaratne et al., 1998), jackfruit wastes (Khuc Thi Hue et al., 2008) or mulberry leaves (Liu et al., 2002).
Grasses such as Napier grass (Pennisetum purpureum) are also potential protein supplements (Wittayakun et al., 2005; Kusmartono, 2007; Ngo Van Man et al., 2001).
Costlier protein-rich feeds such as rapeseed meal (Liu et al., 2002), soybean meal (Odai et al., 2002) and fish meal may also be used (Khan et al., 1990).
Supplementary energy sources include maize silage (Liu XiaoHui et al., 2006), commercial concentrate mixtures (Bhaskar et al., 1992), sugarcane molasses (Nguyen Xuan Trach, 2004), or groundnut haulms (Fayomi et al., 2004). Supplementation may be combined with mechanical, chemical and pressure treatments.
Rice straw fed alone proved to be sufficiently nutritious to feed draught cattle during short periods, but did not meet full animal requirements and had to be supplemented with crude protein and minerals, especially P and Ca (Jackson, 1979).
In dairy cows fed rice straw, legume protein sources were found to have positive effects on DM intake (Oddoye et al., 2002), milk yield and milk composition (Akbar et al., 2000). Higher feed intake and rice straw digestibility may also be obtained when adding maize silage to rice straw (Liu XiaoHui et al., 2006), or after chemical treatment of the straw. Those treatments (see Processes above) improved rumen degradability, total digestible nutrients and N retention in cows (Shen HengSheng et al., 2002; Prasad et al., 1998; Ha et al., 1994). Combination of chemical treatments with grass or protein supplements also offers a wide range of improved diets for cows with positive effects on dairy production and feed costs (Wittayakun et al., 2005; Oddoye et al., 2002; Ngo Van Man et al., 2001; Bhaskar et al., 1992; Khan et al., 1990).
In dairy heifers, a combination of urea-treated rice straw with cottonseed cake resulted in optimal weight gain and the lowest feed costs. Rice straw mixed with rice bran and urea-molasses improved live-weight gain, weight of calves and milk yield (Hari Singh et al., 2001).
In cows and heifers, ensiling rice straw was beneficial to daily milk yield (Chen XiLing et al., 1995) and fat milk content (Ngo Van Man et al., 2001). However, ensiling and fermentation needs caution because damp rice straw has lower DM and overall nutrients, is prone to moulds and possibly aflatoxin contamination in milk (Bhuiyan et al., 2003).
In steers, chemical treatments of the rice straw were also shown to increase feed intake and animal performance (Nguyen Xuan Trach, 2004; Cardoso et al., 2004).
Supplementation with cassava hay, sugarcane molasses or wet brewers grain also resulted in higher feed intakes and improved digestibility (Vongsamphanh et al., 2004; Nguyen Xuan Trach, 2003). However soy sauce residue as a supplement decreased DM intake (Maliwan et al., 2009).
Urea treatments combined with protein supplementation did not give better results than supplementation alone (Nguyen Xuan Trach, 2003).
Wood ash or polysorbate 80 (Tween 80®) treatment added to protein supplementation improved DM disappearance and overall nutritive value (Laswai et al., 2007; Lee et al., 2007).
Every treatment or supplementation of rice straw in sheep was reported beneficial on feed efficiency and animal performance (Kusmartono, 2007; Oddoye et al., 2005; Premaratne et al., 1998; Abdella et al., 1998; Damasceno et al., 2000; Orden et al., 2000; Harada et al., 1999; Rajeev Pradhan et al., 1996). Only one trial reported a supplement of mature poplar leaves to have no beneficial effect on rice straw value (Paliwal et al., 1993).
Ensiling rice straw with Ruminococcus albus and Clostridium cellulovorans increased digestibility and nutritive value (El-Galil, 2008). Lactobacillus plantarum improved total digestible nutrients, digestible energy and dry matter intake (Xu Chun Cheng et al., 2006). Other attempts were less successful: fermenting rice straw with Flammulina velutipes had no effect on Hanwoo sheep intake or performance (Shinekhuu et al., 2009). As mentioned above, ensiling rice straw with mature poplar leaves did not ensure sheep maintenance (Paliwal et al., 1993).
Combining treatment and supplementation gave positive results, as reported in the following table.
Rice straw treatments and supplementation in sheep
Supplementing rice straw with cassava leaves or apple pomace and/or treating rice straw gave better intake, N retention and daily weight gain (Ho Quang Do et al., 2002; Jo IkHwan et al., 2002).