Siratro is used as fresh forage and hay in various ruminant production systems, typically to supply protein in pasture and roughage-based diets of low nutritive value. Unlike other legume forages, siratro has not been reported as causing bloat (Walker, 1977).
Digestibility and nitrogen value
The OM digestibility for fresh siratro forage and for siratro hay is relatively low, in the 51-59% range, which is probably due to the high fibre content. The OM digestibilities of siratro at eight weeks (regrowth, Mero et al., 1998c) and 20 weeks (1st cycle, Mupangwa et al., 2000a) were 54% and 59% respectively. In sacco DM degradability for 8 week-old regrowth hay was 54-58% (the rapidly degradable fraction was 38%) (Mupangwa et al., 1997). In sacco DM degradability changed only slightly from 8 weeks (1st cycle) to 22 weeks (regrowth), from 60 to 62% respectively (Bulo et al., 1994). There is a large difference in nutritive value between the leaves and the stems: in sacco OM degradability (k=0.06) was 65 and 43% for leaves and stems respectively (Mero et al., 1997a).
Siratro protein is quite digestible. Apparent N in sacco digestibility of siratro hay at 20 weeks was 81% in the rumen (48 h incubation) and 95% for the total tract (Mupangwa et al., 2003a). Nitrogen retention in sheep fed only siratro was negative (-8.7 g/d), which may have been caused by an insufficient amount of readily available energy. Rapidly degradable N decreased from 66% at 8 weeks to 37% at 20 weeks, while slowly degradable N increased from 26 to 49% during the same period (Mupangwa et al., 2003c).
Palatability and intake
In a palatability experiment in Australia, cows chose freely autumn siratro over summer siratro, probably because of the higher protein content and digestibility of the former. Growing heifers grazing Setaria sphacelata-siratro pasture selected more siratro when its yield and proportion in sward was high: siratro accounted for 62-73% of the intake whereas its proportion in the pasture was 41-51% (Stobbs, 1977). In a pasture with Dichanthium aristatum (75%), Desmodium incanum (14%) and siratro (4%), lactating cows with calves consumed almost all the siratro during the dry and wet season (Manteaux et al., 1991).
In Tanzania, when offered as sole forage to male sheep (30-31 kg LW), OM intake of siratro was 48-52 g/kg LW0.75 after 8 weeks regrowth, lower than for other legumes (Neonotonia wightii, Stylosanthes scabra and Macrotyloma axillare). Increasing feeding level up to 3 to 4% LW allowed the selection of a more leafy diet with a significant improvement in OM intake (Mero et al., 1998c). In Zimbabwe, also in 31 kg sheep, OM intake was 43.3 g/kg LW0.75 after 20 weeks growth, comparable to the OM intakes for Lablab purpureus and Stylosanthes guianensis (Mupangwa et al., 2000a).
Effect on performance
The effects of feeding siratro as a supplementary source of protein have been studied in dairy cattle, beef cattle, sheep and goats, with generally positive results for both milk yield and growth when compared to roughage-only diets. The following table presents examples of trials involving siratro.
Effect of feeding siratro on the performance of cattle, sheep and goats fed fresh siratro:
(6-7 kg milk/d)
|Siratro included in tropical pasture
||Increased milk yield by 2 kg/d
||Dairy Holstein zebu cows
||Siratro supplementing sugarcane forage
||Can be used as nitrogen supplement to obtain more than 6 L milk/d
||Iriondo et al., 1998
||Pasture sown with siratro and Setaria sphacelata
||Low stocking rate (1.1-1.7 head/ha): high annual weight gain (100-166 kg/head for 20 years) and good reproductive performance (gestation);
High stocking rate (2.3 head/ha): low weight gains (10 kg/head for 10-12 years) resulting in low conception rates.
|Jones et al., 2003
||Young growing steers (131 kg)
||Siratro leaves (32%) with cassava roots (26%) and Cenchrus ciliaris hay of low nutritive value, ad libitum
||Increased hay OM intake (19.1 vs. 21.4 g/kg LW) and nitrogen balance (0.09 vs. 0.27 g/kg LW0 .75), but not DM or OM diet digestibility. Efficiently replaced fishmeal in terms of OM intake (21.4 vs. 20.2 g/kg LW) and nitrogen balance (0.27 vs. 0.32 g/kg LW0 .75)
||Mero et al., 1998d
||Young growing steers (158 kg)
||Siratro leaves (15%) with cassava roots (26%) and Cenchrus ciliaris hay of low nutritive value, ad libitum
||Did not improve hay intake, DM or OM diet digestibility.
||Mero et al., 1998a
||Siratro associated with grass in pasture in different places and at different stock rates
||Average daily weight gain of 0.40 up to 0.58 kg/d
||Lambs (19 kg)
||Chloris gayana pasture (92-96%) with Siratro (8-4%) at three stocking rates: 30, 20 and 10 heads/ha
||Low weight gain (10 to 30 g/d) due to poor quality pasture and insufficient siratro supplementation
||Lemma et al., 2006
||Lambs (23-24 kg)
||Siratro hay (25%) in low quality (2.3% protein) Hyparrhenia pasture
||Increased DM intake of pasture, and DM (34 to 42%) and OM digestibility (40 to 46%) of the whole diet
||Matizha et al., 1997
||Lambs (17.5 kg)
||Siratro hay mixed (31%) to chopped dry maize stover
||Increased diet DM intake and reduced weight loss to maintenance level
||Undi et al., 2001
||Goat kids (3-4 months, 12 kg)
||Siratro (pods maturation, 32%) with Chloris gayana hay ad libitum and crushed maize
||Reduced hay DM intake (134 vs. 182 g/d), but increased daily weight gain (19.2 vs. 6.3 g/d), DM (51 vs. 32%) and OM digestibility (54 vs. 43%)
||Mupangwa et al., 2000b
||Male goats (17-22 kg)
||Chopped siratro hay (30% of the DM requirement) supplementing natural pasture and fresh chopped Napier grass
||Maintained liveweight for 84 days, whereas animals not fed siratro lost 24 g/d
||Njarui et al., 2003
||Goat kids (9 or 13 kg)
||Siratro pasture compared to pure pangola (Digitaria eriantha) pasture (35 d regrowth)
||13 kg kids: better average daily gain (76.2 vs. 59.2 g/d) ;
9 kg kids grazing the same paddocks: no differences observed because siratro almost disappeared due to the previous heavy stocking rate (900 to 1400 kg/ha).
|Alexandre et al., 1989