Does Stocking Rate Manipulation Promote Pasture Sustainability in the Humid Tropics?

Stocking rate manipulation was examined as a means of improving plant diversity (as a measure of pasture sustainability and forage value) in a native grass pasture used for dairy production in the humid tropics of Veracruz, Mexico. Given that environmental impact reduces biotic diversity, plant phylogenetic and functional diversity should decline with increased stocking rate. Stocking rates of 2, 3, and 4 cows · ha^?1 and a rotational grazing plan of 3 d of occupation and 27 d of rest per pasture were applied continuously over 5 yr. Across 200 quadrats in each of two replicate paddocks per treatment, observed species richness, phylogenetic diversity (average taxonomic distinctness based on species presence/absence), and functional diversity (life-cycle duration and growth habit) were assessed. Most species were forb/herbs and forb/herb-subshrubs. Perennial species declined with increased stocking rate (F = 16.36, 0.05 > P > 0.02) while annual-perennial species increased (F = 76.88, 0.01 > P > 0.005); the proportion of annual species was least prominent and did not differ significantly. Observed species richness and phylogenetic diversity did not differ significantly with stocking rate. The correlation between functional diversity for life-cycle duration and phylogenetic diversity was significant and positive, suggesting that plant communities were predominantly assembled randomly from the surrounding species pool rather than through interspecies interactions acting to naturally filter immigrant species, thus leading to more opportunistic and undesired species. Although grazing pressure was not sufficient to alter indices or production measures, they did reveal shifts that may precede further pasture decline, indicating pasture sustainability was not being achieved. These rapid assessment methods permit monitoring for early warnings of reductions in pasture sustainability and forage quality for cattle.     

Live-weight gains of Holstein × Zebu heifers grazing a <Emphasis Type="Italic">Cratylia argentea</Emphasis>/Toledo-grass (<Emphasis Type="Italic">Brachiaria brizantha</Emphasis>) association in the Mexican humid tropics

The legume Cratylia argentea associated to Brachiaria brizantha—Toledo-grass (Bb + Ca) and Toledo-grass alone (Bb) were evaluated under grazing conditions by Holstein × Zebu heifers. Three evaluation periods during three consecutive years, were performed. We measured, daily live-weight gains of heifers (DWG, g/day); biomass dry matter (BDM, kg/ha) at beginning and end of each grazing period. On plant samples, were measured percentages of crude protein (CP), neutral detergent fiber (NDF), acid detergent fiber (ADF), lignin, in situ DM degradability (ISDMD), and voluntary biomass DM intake (VBI, g/LW^0.75) using the Cr₂O₃/in situ indigestibility technique. A completely randomized design was applied, using heifers as experimental units for DWG gains and VBI. At the beginning of the grazing, the BDM for Bb + Ca and Bb–considering the three periods-averaged, 3065 and 936 kg/ha, respectively. Crude protein content of C. argentea was around 20 %; and for Bb + Ca or Bb, ranged from 8.7 to 4.6 %. In either treatment, in situ DM degradability averaged 72.6 % (P ≥ 0.05), regardless of the botanical component type. Biomass DM intake was 83.02 (Bb + Ca) and 89.22 (Bb) g/LW^0.75. The DWG gains (g/heifer) for Bb + Ca and Bb were (per period): 829 and 574 (first); 469 and 118 (second); and 534 and 508 (third). This study showed that the Bb + Ca association was better to improve the daily gain of F1 Holstein × Zebu heifers, as compared to Bb alone. Also, C. argentea associated to a low to medium-quality grass improved the nutritional value of the diet without affecting the biomass DM intake.     

Postpartum follicular development in Brahman cows under two stocking rates

The aim was to study postpartum follicular population dynamics in Brahman cows in the Mexican humid tropics and under different stocking rates. Lactating adult cows (n = 104) grazed native grammas (Paspalum spp., Axonopus spp.) under two stocking rates: 2.8 (high) or 1.7 (low) cows/ha. Follicle numbers and their size (millimetres) were measured by ovarian ultrasound scanning. Animals detected in oestrus and inseminated were taken out of the study. Both stocking rates produced similar (P < 0.05) body condition scores (BCS) and follicular dynamics. The percent of follicles, averaged over stocking rates, of <4 mm diminished and those of 5–8 mm increased with time, whilst those of >9 mm remained constant; respective values for the previously mentioned sizes were 60 days postpartum (n = 101), 49%, 56% and 5%; 120 days (n = 70), 28%, 66% and 6%; and 180 days (n = 15), 9%, 79% and 12%. There was a positive correlation between BCS and follicle number for size <4 mm (r = 0.73, P < 0.01), whilst for size 5–8 mm, this was negative (r = −0.51, P < 0.05), for follicles >9 mm, no difference was found (r = −0.08, P > 0.05). It was concluded that stocking rate did not affect follicular population dynamics of Brahman cows, but BCS dictated the number and categories of follicles present regardless of the stocking rate utilised.     

Segmented regression to describe cumulative milk production of grazing dual-purpose Holstein-Zebu cows

The objective of this study was to compare the fit of seven functions to cumulative daily milk yield records of grazing F1 (Holstein × Zebu) cows in a dual-purpose cattle production unit of the Mexican tropics. Fifty-seven lactations from cows that calved from 1998 to 2001 were used. The functions were quadratic without intercept, three with two segments (both segments linear, the first segment quadratic and the second linear, and both segments quadratic), and three classical growth functions (Gompertz, logistic, and Richards). The Akaike information criterion corrected (AIC_C) was used as criterion of fit, being the function with the best fit the one with the lowest AIC_C value. The best fit was for the segmented function with both segments quadratic, followed closely by the Richards function. The derivatives of these functions give the daily milk yield curve (kg/cow/day), so the former results in a straight line per segment and the latter in the usual shape of the typical lactation. However, as cumulative records produce a monotonic increasing line, neither function can distinguish a priori the presence of a lactation peak. For this reason, it is advisable to examine the common dispersion plot of daily milk yield of each cow, and if a peak is not evident, then proceed to fit the segmented function; otherwise, the function of Richards should be used. The need to study the causes for the absence of a lactation peak in tropical dual-purpose cows is highlighted.