AbstractTwo-year grazing trials were conducted at the Corozal Agricultural Experiment Substation to determine the availability of five grass species: Tanner (Brachiaria radicans), Signal (Brachiaria decumbens), Pangola (Digitaria decumbens), Digitaria milanjiana, and Guinea (Panicum maximum), as forage sources for growing Holstein heifers. Species effects were non-significant in average daily gain (ADG) (range .43 to .51 kg). Digitaria milanjiana was best in ADG, but Signal grass was highest in estimated carrying capacity, 5.87 versus 5.2 to 5.7 head/ha/year for the other four grasses. Variations in ADG were large, both within grasses and within months. ADG was approximately 50% lower from December to January because of temperature effects on grass growth. All five grasses were deemed acceptable for heifer growth, but Tanner grass is not recommended because of possible toxicity. Dry matter (DM) content was highest from April to September but crude protein (CP) was lowest in this period. The DM content varied from <20 to >30%, thus showing its influence on intake. In July and August, the three grasses averaged <7% CP, which may have also affected rate of intake. Under normal temperature and rainfall, maximum temperature had a significant (P = .01) positive correlation with DM content (.64 to .87), but a negative correlation with CP content (- .26 to -. 79). However, abnormal rainfall had a marked effect on these relations. Correlations of OM or CP contents in harvested samples with ADG were variable, indicating that these measures are relatively unreliable for predicting rate of gain. Estimated intake of DM as % body weight ranged from 2.9 to 3.1%. On the average, <6% of available pasture grass DM was utilized. Considering costs, such a low rate of use is not economical; hence, the need for further research on management of fertilized tropical grass pastures for most efficient use of available biomass.
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