Effect of stocking rate on biomass variation and lamb performances for barley stubble in Tunisian semi arid region and under conservation agriculture conditions

The integration of livestock in the practices of conservation agriculture (CA) was assessed in Tunisian
semi-arid conditions. Forty five Barbarine lambs (aged 220 ± 10 days, average body weight 20 ± 2.5 kg)
were used in a performance trial, carried out in the experimental station of INRAT. During the
experiment, lambs were grazing on a plot of barley stubble cultivated according to CA was divided into
6 fenced equal subplots and to each subplot was assigned a stocking rate of animals (15 and 30 lambs
per hectare, SR15 and SR30 respectively). The biomass of stubble and its botanical composition were
estimated 2 times, using quadrats sampling technique. Live weight was determined 3 times after the
start of the experiment (three 15-days successive periods) to calculate live weight gain (LWG) and daily
live weight gain (DLWG). The amount of biomass varied (P <0.05) from 2204 to 2067 kg DM / ha for SR15
plots and from 2404 to 1826.5 kg DM/ha for SR30 ones. This decrease was higher with SR30 (P<0.05).
Heads proportion decreased first, then leaves and finally stems. Biomass chemical composition
declined with sampling period. During the first grazing period, lambs assigned to both treatments lost
LW (P<0.001), mainly SR30 lambs as compared to SR15 (P<0.05, -610 and -110 g, respectively). The
same trend was observed in DLWG (P<0.05). In the second period, the two groups exhibited similar
LWG (about 2 kg) and DLWG (about 171 g/d). In the third period, SR15 lambs maintained their body
weight, while SR30 group lost (P<0.001) about 400 g comparatively to the second period. It was
concluded that under the studied feeding system, stubble grazing without supplementation allowed
Barbarine sheep to maintain body conditions.