Supplement feed efficiency of growing beef cattle grazing native Campos grasslands during winter: a collated analysis

Abstract

Supplementing growing cattle grazing native subtropical Campos grasslands during winter improves the low, even negative, average daily weight gain (ADG) typical of extensive animal production systems in Uruguay. Nonetheless, to render the practice profitable it is crucial to control supplement feed efficiency (SFE), that is, the difference in ADG between supplemented and control animals (ADGchng) per unit of supplement dry matter (DM) intake. Little has been studied specifically on how SFE varies in these systems. The objective of this study was to quantify the magnitude and variation in SFE of growing beef cattle grazing stockpiled native Campos grasslands during winter and assess putative associations with herbage, animals, supplements, and climatic variables. We compiled data from supplementation trials carried out in Uruguay between 1993 and 2018, each evaluating between one and six supplementation treatments. The average ADG of unsupplemented and supplemented animals were 0.13±0.174 and 0.49±0.220 kg/animal/day, respectively. In both cases, ADG decreased linearly as the proportion of green herbage in the grazed grassland was lower, but the ADG of unsupplemented animals was further reduced when winter frosts were numerous. Estimated SFE were moderately high, with an average of 0.21±0.076 ADGchng/kg DM, resulting from average ADGchng of 0.38±0.180 kg/animal/day in response to an average supplementation rate of 1.84±0.68 kg supplement DM intake/animal/day (0.86±0.27 %body weight). No association was found between SFE and supplementation rate nor type (protein vs. energy-based; p>0.05), but forage allowance negatively affected it, and herbage mass positively affected it, yet in a smaller magnitude, suggesting that a balance is needed between the two to maximize SFE. Weather conditions during trials affected SFE (p<0.05), with greater SFE in winters with lower temperatures and more frosts. Daytime grazing time was consistently lower in supplemented animals compared to their unsupplemented counterparts, whereas ruminating time during the day was similar, increasing as the proportion of green herbage decreased. Herbage intake estimated from energy balance suggested the existence of some substitution effect. This agrees with the moderately high SFE and with the total digestible nutrients-to-protein ratio of these subtropical humid grasslands being higher than in semiarid rangelands and dry-season tropical pastures but lower than in sown pastures.

Beef Cattle Grazing Native Grasslands May Follow Three Different Supplement Response Patterns

“…This SFE is defined as the difference or change in average daily gain (ADG) between supplemented (S) and control (C) animals (ADGchng, kg) per unit (kg) of supplement dry matter (DM) intake. In the preceding paper of this same series [6], the authors found an average SFE of 0.21 ± 0.1 ADGchng/kg supplement DM intake, from growing beef cattle grazing native grasslands in winter.…”

“…Data were gathered from 15 supplementation trials carried out between 2004 and 2018 in which growing beef cattle grazing native Campos grasslands in Uruguay were supplemented during late autumn and winter. More details of the database compilation and the response and auxiliary variables of the database can be found in Cazzuli et al [6]. In this paper, only some of the trials (15) collated by these authors were used, because not all of the original 15 trials presented data in such detail that more than two phases were identifiable (i.e., only initial and final BW were available).…”

“…These two variables must be operating to some degree in determining SFE. In fact, Cazzuli et al [6] found that, in these native grasslands, a balance is needed between FA and herbage mass to optimise SFE. In addition, these authors suggested the existence of some magnitude of substitution, further supported by the fact that unsupplemented animals spent more time grazing then their supplemented counterparts.…”

The defoliation dynamics of a stockpiled native grassland pasture follow similar patterns between supplemented and unsupplemented beef calves

“…Therefore, it is unclear to what extent and on which variables does supplementation affect animal selectivity and sward structure, particularly on native grasslands. Particularly, little is known about these processes when animals graze stockpiled herbage of grasslands that combine cool and warm season species and thus constitute horizontally and vertically heterogeneous mixtures of green and dead tissue (Cazzuli et al, 2023; Núñez et al, 2022).…”

“…Therefore, it is unclear to what extent and on which variables does supplementation affect animal selectivity and sward structure, particularly on native grasslands. Particularly, little is known about these processes when animals graze stockpiled herbage of grasslands that combine cool and warm season species and thus constitute horizontally and vertically heterogeneous mixtures of green and dead tissue (Cazzuli et al, 2023;Núñez et al, 2022). Some studies of foraging behaviour of ruminants have been carried out on native grasslands of the Pampa Biome (Da Trindade et al, 2012Gonçalves et al, 2009), but no studies have been conducted aiming to characterize successive grazing strata, nor its interaction with concentrate supplementation.…”