Arkansas - Agricultural Communication Services - University of ...

Arkansas Animal Science Department Report 2001
season was divided into three periods by forage types:
February through April (winter annual period), May through
June (transition period), and July through September (warmseason-grass
period). Stocking rate effects were analyzed as
linear and quadratic polynomial contrasts. Time effects (periods
and year) were analyzed as split plots over time with period
as the subplot and year as the sub-subplot. Period differences
were analyzed as orthogonal contrasts.
Results and Discussion
For the year 2000 overall, there were no differences in
animal performance between the C and R systems (Table 1).
Stocking rate was linearly related to final body weight, ADG,
gain per calf, and gain per acre. During the winter annual
period, ADG and gain per acre were linearly related to stocking
rate, and there was no difference between grazing systems.
During the warm-season grass period, ADG did not differ
with stocking rate or system, but gain per acre increased
linearly with stocking rate.
There was a system x stocking rate interaction for ADG
during the transition forage period when pasture composition
was changing from cool to warm-season grasses in 2000. As
was observed in 1999, transition pastures contained primarily
winter annual for R and contained a higher proportion of
warm-season grass for C (data not shown). At the MED and
HIGH stocking rates, ADG and gain per acre were higher on
R pastures than on C pastures in 2000. In contrast, cattle performed
better on C than R pastures during the transition period
in 1999 (Cassida et al., 2000), an effect attributed to poor
forage quality of over-mature winter annual pasture on R
treatments. In 2000, excess forage was cut as hay about 6
weeks earlier (mid-April versus late May) than in 1999. This
prevented R pastures from becoming as mature as they had in
1999 and probably kept higher quality winter annual forage
in front of the R cattle in 2000. Forage quality samples are
being analyzed to investigate this.
Pooled year data were analyzed in all possible subsets
to investigate the source of interactions. All animal performance
variables showed significant four-way interactions (system
x stocking rate x period x year, P < 0.05). The most consistent
finding was a positive linear relationship between
stocking rate and gain per acre in transition and warm-season
grass periods, and for the grazing season as a whole. Among
periods, benefits for R versus C grazing systems were only
found at MED or HIGH stocking rates in the winter annual
(1999) or transition (2000) periods. Since winter annuals
were the major component of R pastures in both of these periods,
it is possible that the same factors accounted for results
in both years. In 2000, spring grazing began about three
weeks later than in 1999, which shortened the duration of the
winter annual period. If the better pasture management
described above for 2000 also improved forage quality during
transition period, the combined effect may have been to push
the benefit of the R treatment forward into the transition period
in 2000.
In 2000, hay feeding was required on C-MED, C-
HIGH, and R-HIGH pastures to cover forage shortages. In
2000, more hay was baled on R than on C pastures (P < 0.05),
with some hay harvested from all R stocking rates (Table 1).
More hay was fed than was baled on C-MED and C-HIGH
pastures. There were no interactions with year in the pooled
year hay analysis. Averaged across years, more hay was fed
on C than R pastures (0, 875, 2106, and 0, 0, 471 lb hay
DM/acre, P < 0.05, for LOW, MED, and HIGH on C and R
pastures, respectively). Less hay was baled on C than R pastures
(3761, 270, 0, and 4512, 2507, 1622 lb hay DM/acre).
Across years, amount of hay fed and harvested was linearly
related (P < 0.01) to stocking rate for both systems. Less (P <
0.05) hay was harvested in 2000 than in 1999 (Cassida et al.,
2000) as a result of cutting hay at earlier maturity (hence
lower yields) and drought. Less (P < 0.05) hay was fed on C
pastures in 2000 than in 1999.
Implications
Rotational stocking at medium to high stocking rates
may produce better ADG, gain per acre, and hay yield than
continuous stocking when calves graze pastures that contain
primarily winter annuals. On warm-season-grass pastures,
grazing system does not affect calf performance. Stocking
rate has a greater effect on calf performance than grazing
system.
Literature Cited
Cassida, K., et al. 2000. Ark. Agri. Exp. Sta. Res. Ser. 478:61.
128