Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/1/2011
Publication Date: 8/29/2011
Citation: Burns, J.C., Fisher, D.S., Pond, K.R. 2011. Flaccidgrass forage mass and canopy characteristics related to steer digesta kinetics and intake. Crop Science. 51:2897-2903. Interpretive Summary: The characteristics of a pasture can greatly influence how animals select their diet and can alter animal daily performance. Flaccidgrass, an introduced perennial, warm-season grass has not been well studied relative to canopy shifts with changes in forage mass (FM) and subsequent animal responses. In this study canopy height ranged from 17 cm for the Short FM to 52 cm for the Tall FM treatment with an associated linear increase in FM from 1245 to 3585 kg ha-1. This range in FM produced canopies that had greater proportions of stem and reduced proportions of dead tissue while the proportion of leaf was not altered and averaged 24.3% of the FM. The whole canopy in vitro dry matter disappearance (IVDMD), an estimate of forage digestibility, was similar (549 g kg-1) among FM treatments with the leaf component greatest in IVDMD (654 g kg-1), followed by stem (585 g kg-1) and dead tissue (406 g kg-1). The IVDMD increased linearly in dead tissue from 380 g kg-1 for Short to 421 g kg-1 for Tall. Steers, however, managed to select a diet that averaged 713 g kg-1 in IVDMD and 486 g kg-1 in NDF with no difference either among FM treatments or between early and late season. This was reflected in difference values (masticate composition minus canopy composition) for IVDMD and NDF which were similar among FM treatments and between early and late season, averaging 224 g kg-1 greater and 116 g kg-1 lesser, respectively. Dry matter intake and digestible DMI estimates, associated with leaf nutritive value, were also similar among FM treatments averaging 1.99 and 1.29 kg 100-1 k BW d-1 respectively, with digestible DMI being greater in early than in late season (1.29 vs. 1.18 kg 100-1 kg BW d-1). These data indicate that despite large FM differences, steers were able to select diets that were similar across a range of FM characteristics that varied both in quantity and in morphology. Consequently, flaccidgrass can provide a similar nutritive value under varied plant canopies with limitations to per hectare animal production mainly associated with plant regrowth potential and control of stocking rate. Animal daily responses and animal gain ha-1 and associated stocking rate remains to be addressed.
Technical Abstract: ‘Carostan’ flaccidgrass (Pennisetum flaccidum Griseb.) has potential as a perennial, warm-season grass for pastures. This 2-yr study examined digesta kinetics, dry matter intake (DMI), and diet of steers as related to canopy characteristics. Steers (Bos Taurus L.) were continuously and variably stocked at canopy heights of Short (15 to 20 cm), Medium (30 to 40 cm), and Tall (45 to 55 cm) with forage mass (FM) of 1245, 2600, and 3585 kg ha-1 , respectively (linear; P < 0.01). Neither mean retention time nor passage rate of either liquid or digesta phases were altered by FM. Fecal output increased linearly (P = 0.04) from Short to Tall [0.643 to 0.739 kg 100 -1 kg body weight (BW)] indicating a possible variation in DMI. However, DMI and digestible DMI (based on leaf IVDMD) were similar among FM (mean = 1.99 and 1.29 kg 100-1 kg BW). Leaf composed 24.3% of the three canopies and was not altered by FM. Stem increased linearly (P = 0.01) from 40.1 to 53.1% with height, whereas dead tissue declined linearly (P = 0.06) from 34.8 to 23.4%. Steers selected diets (1-yr data), regardless of FM, similar in IVDMD (mean = 713 g kg-1) and NDF (mean = 486 g kg-1). In general, steer digesta kinetics were not influenced by FM as animals adjusted grazing behavior to result in similar responses. Flaccidgrass canopies were flexible and no particular optimum FM was detecting for an optimum diet. The influence of FM on pasture productivity (regrowth after defoliation) requires additional study.