Composition and nutritive values
Energy is a major nutritional limitation of kikuyu due to its high cell wall content and to a low digestibility of structural components. The deficiency of non-structural carbohydrates is likely to be aggravated by diurnal fluctuation in the plant (Marais et al., 1990). The concentration of metabolizable energy is low, 8.5 MJ/kg DM, as calculated from an OM digestibility of 65% (Marais, 2001). Recorded OM digestibility varies between 47 and 73%, depending on the stage of regrowth (Feedipedia, 2010). Due to the large difference between leaf and stem tissue, the nutritive value appears to be optimized at the 4.5 leaves per tiller growth stage (Marais, 2001).
Crude protein varies from 8.5 to 25.6% DM (Murtagh, 1990) and is higher than that of other tropical grasses. However, the high nitrogen levels in kikuyu induce a poor protein metabolism and low animal productivity (Marais et al., 1990; Hanna et al., 2004; Carvalho et al., 2010). A high concentration of nitrogen in young kikuyu regrowth may result in high rumen ammonia, which is largely lost as urea via the urine. The NDF content ranges from 58.1 to 74.1% DM and resembles temperate species in having relatively low NDF, but digestibility is similar to those of other tropical species (Marais, 2001). There is a negative correlation between cell wall content (NDF) and digestibility (Moore et al., 1972; Feedipedia, 2010).
Kikuyu appears not to contain condensed tannins which could reduce ammonia formation in the rumen (Jackson et al., 1996; Marais, 2001). Lignin content in kikuyu can be as high as 6% DM in rich stems at the end of the season (Marais, 2001).
Animal production on kikuyu should be improved by an energy supplement. Poor absorption of Ca and Na by kikuyu also makes supplementation essential for these two minerals for optimal animal production on kikuyu pasture (Marais, 2001).
For grazing dairy cows, kikuyu, despite not being available in winter, was one of the most preferred grasses, after prairie grass (Bromus wildenowii), among 14 other grasses and legumes, including white clover, in 8 seasons over 2 years (Horadagoda et al., 2009). The relative palatability can be reasonably predicted by the water soluble carbohydrates and nitrate-nitrogen contents (Soto et al., 1980; Dugmore et al., 1991; Marais, 2001).
Due to its high DM yield, kikuyu pasture supplemented with energy has been used for dairy cattle in many countries. In Friesian cows, milk production was restricted to 13-16 L/d even on well-managed kikuyu pasture (Reeves et al., 1996; Marais, 2001; Hamilton et al., 1992; Carvalho et al., 2010). The low milk protein content (less than 3.0%) indicates low efficiency of nitrogen utilization (average 17.4%) for the synthesis of milk proteins. The high milk content of conjugated linoleic acid (20.0 mg/g lipids) showed the high linoleic and linolenic acid content of kikuyu (Correa et al., 2008). The supplementation of dairy cows grazing rotationally kikuyu with high energy concentrates is sufficient for mid and late lactation, without requiring protein supplementation. Milk production may reach an average of 15.7 kg/cow/day (Semmelmann et al., 2008).
The average potential milk production from cows grazing Kikuyu was restricted at 12 L/cow/day, even though the potential of this forage was 29 L/cow/d given its energy content (Correa et al., 2008). With levels of supplementation between 20 to 100% of requirements for milk production, a linear increase in milk yield of grazing cows was up to 24 kg of milk/day (Carvalho et al., 2010). In a survey of 229 dairy farms in Colombia, 33% of the farms had cows producing 16 to 20 L/day (Osorio, 2004). The milk yield of Jersey cows grazing kikuyu pasture in late summer and receiving a maize-based supplement (6 kg/d), or a maize/fishmeal supplement (8% fishmeal in an iso-energetic diet) increased with the level of fishmeal fed until 19.5 kg/day (Malleson et al., 2009). The total milk production may be increased when kikuyu is over-sown with annual ryegrass (from 3.9 T milk/ha to 8.1 T milk/ha) or white clover (7.3 T milk/ha). But the mean annual grazing capacity has to be considered to evaluate the effective gain in milk production per hectare. Indeed, the milk produced may vary between seasons, being higher during spring and summer (15 kg/d and 14.4 kg/d) than during autumn (12.1 kg/d) (Botha et al., 2008).
By adjusting the loading or the ratio of days grazed per day rested and taking into account the grazing period, it was possible to improve the carcass weight of heifers grazing on an irrigated kikuyu/white clover pasture (Clatworthy et al., 1980).
There are conflicting results about the performance of sheep grazing kikuyu forage, due to differences in the quality of herbage as influenced by the rest period and leaf proportion (Rethman et al., 1973; Barnes et al., 1993). The potential of kikuyu forage to meet the requirements of dry ewes without supplementation appears limited: grazing ewes lost weight, varying from 4.7 to 10.3% of the initial body weight during the 8-week grazing periods over four seasons (de Villiers et al., 2002).
The combination of kikuyu pastures with trees belts of Tasmanian blue gum (Eucalyptus globulus) led to significant increases in the clean wool production of Merino sheep (75 kg/ha) compared to kikuyu alone. Kikuyu is tolerant of shade and interacts positively with trees for soil nutrients and periodic water stress. This combination allows for substantial reductions in groundwater recharge from irrigation (Sanford et al., 2003).
Increasing the length of the regrowth period of kikuyu from 50 to 90 days, before making hay, reduced its feeding value for lambs due to a lower OM intake, but did not affect digestibility. Hay intake appeared to be mostly limited by its content of structural carbohydrates (Chiesa et al., 2008). In another experiment, between days 39 to 78, voluntary intake of penned sheep increased with herbage maturity, with the highest values at 50 and 78 days of regrowth (66.86 and 70.64 g/kg W0.75/day respectively). Dry matter intake increased due to changes in DM digestibility and fibre contents. Voluntary intake and DM digestibility are negatively correlated with ADF, lignin and Si contents. These parameters are positively correlated with leaf:stem ratio (Soto et al., 1980).
The use of tree leaves can help to improve the nutritive value of kikuyu hay. In Chiapas (Mexico), the leaves of Buddleja skutchii, a common multipurpose fodder tree have been substituted for 50 or 100% of kikuyu hay without decreasing DM intake and digestibility (Camacho et al., 1999). However, a later experiment found that the inclusion of Buddleja leaves maintained daily weight gain as well as wool and manure production, but it decreased DMI and digestibility resulting in lower economic benefits than with kikuyu grass alone (Nahed et al., 2003).