Grazing of cover crops in integrated crop-livestock systems.

Conventional agriculture is specializing rapidly into the management of few monoculture crops, threatening crop diversity and questioning the sustainability of extensive cropping systems. The grazing of cover crops in integrated crop-livestock systems could be a feasible biologically based technology to restore crop diversity and mitigate ecological issues in cropping systems. However, there is limited evidence on plausible synergies or trade-offs for the practice, and about how grazing plans could affect the herbage production and services from cover crops. This work assessed the effects of cattle grazing on the primary and secondary production of annual ryegrass (Lolium multiflorum) in an integrated ryegrass-soybean rotation system. Specifically, the prediction for synergistic effects of cattle grazing on the ryegrass herbage production, residual crop cover and animal performance were tested in a 2-year (2014 and 2015) study comprising a randomized complete block design of four grazing intensity treatments, replicated three times. A no-cattle grazing treatment (NG), used as control, or continuous grazing with Holstein heifers (~220kg live weight) at targeted sward heights of 5, 10, 15 and 20cm (hereafter referred as G5, G10, G15 and G20, respectively) was applied to ryegrass plots. The herbage production and residual herbage cover of ryegrass, and the average daily gain (ADG, kg/day) and live weight gain per hectare (LWG, kg/ha) of heifers were analyzed by ANOVA (P<0.05) and compared by the TukeyHSD test (P<0.05). Regression models were used to estimate relationships between herbage production, animal performance and sward height. The herbage production was 60% higher (P<0.01) for the grazing treatments compared to NG. The residual herbage for G15 and G20 was not different than that for NG and increased linearly as sward heights increased, reaching highest values for G15 and G20. Maximum ADG was 1.10kg/day for ryegrass grazed at a 20.6cm height, whereas maximum LWG was 427kg/ha for ryegrass grazed to a 16.1cm height. The results support the hypothesis for synergistic effects of using annual ryegrass as a dual forage and service cover crop. Moderate grazing intensity to sward height of 12-18cm with continuous stocking led to optimized forage production and utilization by dairy heifers.

Monitoring and assessment of ingestive chewing sounds for prediction of herbage intake rate in grazing cattle.

Accurate measurement of herbage intake rate is critical to advance knowledge of the ecology of grazing ruminants. This experiment tested the integration of behavioral and acoustic measurements of chewing and biting to estimate herbage dry matter intake (DMI) in dairy cows offered micro-swards of contrasting plant structure. Micro-swards constructed with plastic pots were offered to three lactating Holstein cows (608±24.9 kg of BW) in individual grazing sessions (n=48). Treatments were a factorial combination of two forage species (alfalfa and fescue) and two plant heights (tall=25±3.8 cm and short=12±1.9 cm) and were offered on a gradient of increasing herbage mass (10 to 30 pots) and number of bites (~10 to 40 bites). During each grazing session, sounds of biting and chewing were recorded with a wireless microphone placed on the cows' foreheads and a digital video camera to allow synchronized audio and video recordings. Dry matter intake rate was higher in tall alfalfa than in the other three treatments (32±1.6 v. 19±1.2 g/min). A high proportion of jaw movements in every grazing session (23 to 36%) were compound jaw movements (chew-bites) that appeared to be a key component of chewing and biting efficiency and of the ability of cows to regulate intake rate. Dry matter intake was accurately predicted based on easily observable behavioral and acoustic variables. Chewing sound energy measured as energy flux density (EFD) was linearly related to DMI, with 74% of EFD variation explained by DMI. Total chewing EFD, number of chew-bites and plant height (tall v. short) were the most important predictors of DMI. The best model explained 91% of the variation in DMI with a coefficient of variation of 17%. Ingestive sounds integrate valuable information to remotely monitor feeding behavior and predict DMI in grazing cows.

Effect of sward surface height and level of herbage depletion on bite features of cattle grazing Sorghum bicolor swards.

To maximize herbage DMI, pregrazing sward surface height (SSH) and level of herbage depletion (HD) must be such that variables determining short-term herbage intake such as bite mass (BM) and bite rate (BR) are optimized. The objective of this study was to determine a SSH target and the level of HD as a proportion of the SSH that optimizes BM and BR of beef heifers grazing Sorghum bicolor swards. Two experiments were conducted using 2 S. bicolor swards and 4 beef heifers (25 mo old; 322 kg BW). Experiment 1 compared the effect of 6 pregrazing SSH, 30, 40, 50, 60, 70, and 80 cm, on BM, BR, and jaw movements. Experiment 2 assessed the effect of HD level as a proportion of SSH (0.17, 0.34, 0.50, 0.67 and 0.84) on BM, BR, and jaw movements using the optimal pregrazing SSH defined in Exp. 1. Short-term herbage DMI was estimated using a double-weighing technique and corrected for insensible BW loss. Herbage DMI was subsequently used to calculate the BM. Net eating time and jaw movements for apprehension and manipulation + mastication during grazing as well as total jaw movements were determined using the IGER (Institute of Grassland and Environmental Research) behavior recorders. Bite rate and the number of total jaw movements per gram herbage DMI were derived from jaw movement count and measurements of herbage DMI. The results of Exp. 1 showed low and high SSH constraint the ease of herbage harvesting. Greater BM are maintained until a SSH of 50 cm is reached (P < 0.05) and then decline at greater SSH due to herbage dispersion. The nonbiting jaw movement rate increased at greater SSH whereas BR decreased (P < 0.05). For both variables, the turning point was close to a SSH of 50 cm. Experiment 2 showed that such an optimization of BM and BR was maintained until an HD level of 0.34 was reached (P < 0.05). There was a linear increase in both the total jaw movements per unit herbage DMI and the nonbiting jaw movements rate (manipulation + mastication) subsequent to levels of HD greater than 0.34 (P < 0.05). These studies provide, for the first time, sward feature targets to manage grazing and optimize BM and BR, aiming to maximize the short-term herbage DMI of cattle grazing S. bicolor swards.