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Context: Cattle and sheep emit methane, a potent greenhouse gas, as part of the fermentation process of feed digestion in their gut; however, the mechanisms explaining differences among animals in enteric methane production are not fully understood. Aim: To investigate whether variation among animals in their ability to digest their test ration was associated with phenotypic and genetic variation in methane production. Methods: The experiment used 135 Angus beef cattle measured for their phenotypic and genetic merit for methane production. The extent of digestion of the dry matter (DMD) in the test ration by individual cattle was determined using silica as a naturally present indigestible marker. Its concentration in feed consumed and faeces was determined using rapid portable X-ray fluorescence spectroscopy, from which DMD was calculated. Key results: Higher daily methane-production rate (MPR), higher methane produced per unit of feed consumed (methane yield; MY) and higher methane produced than the predicted daily production (residual MPR; RMP) by animals was accompanied by higher DMD. Higher genetic merit for MPR was also accompanied by higher DMD, but DMD had no detectable association with genetic variation in the other two methane emission traits. The regression coefficients for change in MPR (g/day), MY (g/kg DMI), RMP (g/day) with change in DMD (%) were 2.6 ± 1.1 (s.e.; P < 0.05), 0.14 ± 0.07 (P < 0.1) and 0.68 ± 0.38 (P < 0.1) respectively. Conclusions: Differences among animals in their DMD were found and were associated with phenotypic variation in the three methane emission traits studied, and with genetic variation in daily methane production. The results support the caution that feeding and breeding interventions seeking to reduce methane emissions can also reduce the extent of digestion of feed by cattle. Implications: Feeding and breeding interventions that seek to reduce methane emissions may change rumen physiology and reduce the extent of digestion of feed by cattle, which may be undesirable.

The hypothalamus controls feed intake in response to various physical, nutrient and hormone signals that are integrated in the arcuate nucleus (ARC). The ARC contains both orexigenic and anorexigenic neurons which communicate with the theoretical hunger (lateral hypothalamic area; LHA) and satiety (ventromedial hypothalamus; VMH) centres to control feed intake. Cattle in northern Australia have a 35 to 60% reduction in voluntary feed intake during the annual dry season (April to November) due to a very low crude protein [CP; 25 to 40g/kg dry matter (DM)] content of available pasture. This reduction in intake is likely regulated by metabolic mechanisms. It was hypothesised that gene pathways within the ARC, LHA and VMH of steers would be differentially expressed in response to the CP content of the diet and metabolisable energy (ME) intake.

Bos indicus steers (n=15; 194 ± 10kg liveweight, mean ± S.D.) were fed a high CP-high dry matter digestibility (DMD) diet ad libitum to provide unrestricted ME intake (HCP-HDMD-U), a low CP-low DMD diet ad libitum to provide unrestricted ME intake (LCP-LDMD-U) or a HCP-HDMD diet restricted to an equivalent ME intake of the LCP-LDMD diet (HCP-HDMD-R) for 98 days. Intake of steers was significantly different between all treatments (P<0.001; 28.5, 17.0 and 9.7 gDM/kg LW.day for HCP-HDMD-U, LCP-LDMD-U and HCP-HDMD-R respectively). ME intake was significantly higher (P<0.001) for steers fed the HCP-HDMD-U (0.24 MJ/kg LW.day) but was similar for steers fed the LCP-HDMD-U and HCP-HDMD-R (0.07 MJ/kg LW.day) diets.

Steers were euthanised and RNA was extracted from the ARC, VMH and LHA of these steers followed by preparation of cDNA libraries and subsequent sequencing on Illumina NovaSeq 6000 (producing 100 bp single end reads). FASTQ files were trimmed using ‘trimmomatic’ and mapped against Bos taurus reference genome release 9 using HISAT2 (average 56% reads mapped within genes). Within the ARC, 179 and 142 genes were differentially expressed between the HCP-HDMD-U and the LCP-LDMD-U and HCP-HDMD-R treatments, respectively. The VMH and LAT had minimal differentially expressed genes in response to dietary treatments. These data indicate the importance of the ARC in integrating signals in steers with different levels of ME intake.

Sequencing performed by Australian Genome Research Facility, Australia. Bioinformatics performed by QFAB@QCIF, Institute of Molecular Biology, The University of Queensland. We gratefully acknowledge Meat and Livestock Australia for funding this work. D Innes was in receipt of scholarships from The University of Queensland and Meat and Livestock Australia.

The pregnancy rate of heifers affects the efficiency and profitability of beef herds. Heifers extreme in rib fatness (Fat) or post-weaning residual feed intake (RFI) estimated breeding values (EBVs) were evaluated for their pregnancy rates at two locations in the southern agricultural regions of Australia (Struan and Vasse) as part of the Beef Cooperative Research Centre Maternal Productivity Project. Heifers divergent in Fat (High-Fat and Low-Fat) had differences in fat depth pre-joining at the 12/13th rib (4.4 mm vs 3.5 mm) and P8 rump site (6.1 mm vs 4.8 mm). This was associated with significant differences in pregnancy rates over a 9-week joining period (91.5% vs 83.0%) and an even larger difference when calculated over a 6-week joining period (77.3% vs 65.0%). Heifers divergent in RFI (Vasse only) also differed in rib fat (7.6 mm vs 6.4 mm) and P8 fat (11.0 vs 9.2 mm), but not significantly in pregnancy rates between the two RFI (High-RFI and Low-RFI) genotypes following a 9-week (92.4% vs 88.5%) or 6-week (81.2% vs 73.7%) joining period. The phenotypic analysis of the Fat and RFI heifers together indicated that weight and fat depth were the largest contributing factors to variation in pregnancy rates, and age and pre-joining weight gain were not significant. These phenotypic characteristics indicated that producers can manage heifers to particular weight and fat combinations to improve heifer conception rates. Associations of BREEDPLAN EBVs with heifer fertility showed that a shorter days-to-calving EBV had the biggest impact (P < 0.001) on heifer pregnancy rates and rib fat and scrotal size EBVs were close to significant (P < 0.10).

This experiment investigated phenotypic and genetic relationships between carbon dioxide production, methane emission, feed intake, and postweaning traits in Angus cattle. Respiration chamber data on 1096 young bulls and heifers from 2 performance recording research herds of Angus cattle were analyzed to provide phenotypic and genetic parameters for carbon dioxide production rate (CPR; n = 425, mean 3,010 +/- SD 589 g/d) and methane production rate (MPR; n = 1,096, mean 132.8 +/- SD 25.2 g/d) and their relationships with dry matter intake (DMI; n = 1,096, mean 6.15 +/- SD 1.33 kg/d), body weight (BW) and body composition traits. Heritability estimates were moderate to high for CPR (0.53 [SE 0.17]), MPR (0.31 [SE 0.07]), DMI (0.49 [SE 0.08]), yearling BW (0.46 [SE 0.08]), and scanned rib fat depth (0.42 [SE 0.07]). There was a strong phenotypic (0.83 [SE 0.02]) and genetic (0.75 [SE 0.10]) correlation between CPR and MPR. The correlations obtained for DMI with CPR and with MPR were high, both phenotypically (r(p)) and genetically (r(g)) (r(p): 0.85 [SE 0.01] and 0.71 [SE 0.02]; r(g) (0.95 [SE 0.03] and 0.83 [SE 0.05], respectively). Yearling BW was strongly correlated phenotypically (r(p) >= 0.60) and genetically (r(g) > 0.80) with CPR, MPR, and DMI, whereas scanned rib fat was weakly correlated phenotypically (r(p) < 0.20) and genetically (r(g) <= 0.20) with CPR, MPR, and DMI. The strong correlation between both CPR and MPR with DMI confirms their potential use as proxies for DMI in situations where direct DMI recording is not possible such as on pasture.

This study was to examine if it is possible to accelerate sorghum digestion in broiler chickens by targeting fermentation of the xylan. Cobb 500 broilers (n = 960, 80 birds per treatment) were fed 12 sorghum-soybean meal-based dietary treatments fed as 3 phases (starter d 0 to 12, grower d 13 to 23, finisher d 24 to 35), with 8 replicate pens of 10 birds per treatment. For half of the treatments (n = 6), 10% of the sorghum in the diet was directly replaced with 10% wheat bran, as a source of fermentable fibre. The diets were supplemented with either 0, 50 or 2,000 mg/kg xylo-oligosaccharides (XOS), with or without xylanase application. Body weight gain (BWG), feed intake (FI) and feed conversion corrected for mortality (cFCR) was determined at d 0 to 35, and male and female body weight were measured on d 35. On d 35, ileum and caeca samples were collected from 2 birds per pen, for determination of caecal cellulase and xylanase activity, microbiota composition and short chain fatty acid (SCFA) concentration, and ileal XOS concentration. Supplementation with 2,000 mg/kg XOS caused increased BWG at d 0 to 35 (P = 0.007) and enhanced caecal propionic, valeric and succinic acid concentration (P < 0.05). Wheat bran increased FI (P = 0.018) and BWG (P = 0.016), as well as caecal Bifidobacteria concentration (P < 0.001). Ileal XOS concentration was greatest when feeding combined wheat bran, 2,000 mg/kg XOS, and xylanase, resulting in increased caecal total SCFA, acetic acid and butyric acid concentration, and xylanase and cellulase activity (P < 0.05). Results from this study present that feed efficiency in birds fed sorghum-based diets is improved as a consequence of supplementing with fermentable fibre, xylanase and XOS.

The present study investigated whether supplementing fibre-degrading enzymes can ameliorate the severity of subclinical necrotic enteritis (NE) in broiler chickens offered wheat- or maize-based diets. A total of 1,544 mixed-sex broiler chickens were assigned to 16 experimental treatments as a 2 x 2 x 4 factorial arrangement of treatments. The factors were the following: NE challenge, yes or no" diet type, wheat- or maize-based" and enzyme supplementation, control (no enzyme), family 10 xylanase (XYN10), family 11 xylanase (XYN11) or b-mannanase (MAN). Each treatment was replicated 6 times, with 16 birds per replicate pen. A three-way challenge x diet type x enzyme interaction occurred for body weight at 21 d of age (P ¼ 0.025) and overall feed conversion ratio (P ¼ 0.001). In the non-challenged birds fed the wheat-based diet, supplementing MAN increased d 21 body weight compared to the control. In challenged birds fed the maize-based diet, supplemental XYN11 impeded body weight and overall FCR compared to the control. Birds offered the maize-based diet presented heavier relative gizzard weights at both 16 and 21 d of age (P < 0.001) and reduced liveability (P ¼ 0.046) compared to those fed the wheatbased diet. Enzyme supplementation reduced ileal and jejunal digesta viscosity at 16 d of age only in birds fed the wheat-based diet (P < 0.001). XYN11 increased ileal digesta viscosity in birds fed the maizebased diet, and MAN reduced it in birds fed the wheat-based diet at 21 d of age (P ¼ 0.030). Supplementing XYN11 improved ileal soluble non-starch polysaccharides (NSP) digestibility in birds fed the wheat-based diet compared to non-supplemented birds (P < 0.001). Birds fed the wheat-based diet displayed a higher abundance of Bifidobacterium, Lactobacillus and Enterobacteriaceae and butyric acid in the caeca at 16 d of age compared to birds fed the maize-based diet (P < 0.05). In conclusion, supplemental XYN11 exacerbated the negative impact of NE on growth performance in birds fed the maize based diet. Supplementing wheat-based diets with fibre-degrading enzymes ameliorates production losses induced by NE.

Respiration chambers are considered the reference method for quantifying the daily CH₄ production rate (MPR) and CO₂ production rate (CPR) of cattle" however, they are expensive, labor intensive, cannot be used in the production environment, and can be used to assess only a limited number of animals. Alternative methods are now available, including those that provide multiple short-term measures of CH₄ and CO₂, such as the GreenFeed Emission Monitoring (GEM) system. This study was conducted to provide information for optimizing test procedures for estimating MPR and CPR of cattle from multiple short-term CH₄ and CO₂ records. Data on 495 Angus steers on a 70-d ad libitum feedlot diet with 46,657 CH₄ and CO₂ records and on 121 Angus heifers on a 15-d ad libitum roughage diet with 7,927 CH₄ and CO₂ records were used. Mean (SD) age and BW were 554 d (SD 92) and 506 kg (SD 73), respectively, for the steers and 372 d (SD 28) and 348 kg (SD 37), respectively, for the heifers. The 2 data sets were analyzed separately but using the same procedures to examine the reduction in variance as more records are added and to evaluate the level of precision with 2 vs. 3 min as the minimum GEM visit duration for a valid record. The moving averages procedure as well as the repeated measures procedure were used to calculate variances for both CH₄ and CO₂, starting with 5 records and progressively increasing to a maximum of 80 records. For both CH₄ and CO₂ and in both data sets, there was a sharp reduction in the variances obtained by both procedures as more records were added. However, there was no substantial reduction in the variance after 30 records had been added. Inclusion of records with a minimum of 2-min GEM visit duration resulted in reduction in precision relative to a minimum of 3 min, as indicated by significantly (P < 0.05) more heterogeneous variances for all cases except CH₄ in steers. In addition, more records were required to achieve the same level of precision relative to data with minimum GEM visit durations of 3 min. For example, in the steers, 72% reduction in initial variance was achieved with 30 records for both CH₄ and CO₂ when minimum GEM visit duration was 3 min, relative to 45 records when data with a minimum visit duration of 2 min were included. It is concluded from this study that when using records of multiple short-term breath measures of CH₄ or CO₂ for the computation of an animal's MPR or CPR, a minimum of 30 records, each record obtained from a minimum GEM visit duration of 3 min, are required.

Context: Research into improving feed efficiency by ruminant animals grazing pastures has historically been restrained by an inability to measure feed intake by large numbers of individual animals. Recent advances in portable breath measurement technology could be useful for this purpose but methodologies need to be developed.

Aims: To evaluate predictive models for metabolisable energy intake (MEI) by free-ranging cattle using multiple short-term breath samples and then apply these to predict MEI by free-ranging cattle in a historic grazing experiment with cattle genetically divergent for residual feed intake (feed efficiency).

Methods: Predictive models for MEI were developed using bodyweight (BW) data, and carbon dioxide production rate (CPR) and methane production rate (MPR) from multiple short-term breath measurements, from an experiment with long-fed Angus steers on a grain-based diet, and an experiment with short-fed Angus heifers on a roughage diet. Heat production was calculated using CPR and MPR. Energy retained (ER) in body tissue gain by steers was calculated from BW, ADG, initial and final subcutaneous fat depths, and for both groups using feeding-standards equations.

Key results: Metabolic mid-test BW (MBW) explained 49 and 47% of the variation in MEI in the steer and heifer experiment, respectively, and for the steers adding ADG and then subcutaneous fat gain resulted in the models accounting for 60 and then 65% of the variation in MEI. In the steer experiment, MBW with CPR explained 57% of the variation in MEI, and including MPR did not account for any additional variation. In the heifer experiment, MBW with CPR explained 50%, and with MPR accounted for 52% of the variation in MEI. Heat production plus ER explained 60, 35 and 85% of the variation in MEI in the steer and the heifer experiments, and in the pooled data from both experiments, respectively.

Conclusions: Multiple short-term breath measurements, together simple BW data, can be used to predict MEI by free-ranging cattle in studies in which animals do not have feed-intake or ADG recorded.

Implications: This methodology can be used for research into improving feed efficiency by farm animals grazing pastures.

Residual feed intake (RFI) is the difference between an animal's actual feed intake and that which would be expected based on production. This experiment was to test the hypothesis that part of the variation in RFI may be due to differences in energetic efficiency through changes in heat production, these being in part due to differences in protein metabolism. Following three generations of divergent selection for RFI, eight High and eight Low-RFI heifers were fed at both 105% and 180% of predicted maintenance feed requirements. Between-RFI line and feeding-level differences were assessed for energy intake, protein metabolism, heat production, body composition, energy and nitrogen balance and digestibility. The RFI lines did not differ in protein metabolism or heat production. The High-RFI heifers deposited 51% and 56% more subcutaneous fat at the P8 rump and 12/13th rib sites, respectively, with no difference in eye muscle area gain or average daily weight gain. The greater fat deposition of High-RFI heifers was due to a larger ad libitum feed consumption compared with the Low-RFI heifers. Energy and nitrogen balance did not differ between the RFI lines. The energy transactions indicated no difference in the efficiency of energy use on 105% maintenance, although when fed 180% of maintenance the differences in feed intake suggest variation in appetite as the mechanism contributing to RFI. All of the extra energy consumed by High-RFI heifers above maintenance and deposition of protein was associated with additional energy retained as fat. This study suggests that selection for RFI may not lead to improved efficiency of energy use.

Context. Measurement of weight provides the basis of most performance-recording schemes for beef cattle around the world. The limitation of faster growth rate as a breeding objective, without considering changes in mature-cow weight, is the expected increase in cow size and, hence, feed requirements. Aims. To measure the correlated changes in feed intake and efficiency of cows, calves and the cow–calf unit following divergent selection for growth rate. Methods. The cows and their calves came from three lines of Angus cattle selected for either fast weight gain to yearling age (the High-line), slow weight gain (the Low-line), or from an unselected Control-line. Efficiency was evaluated over an annual production cycle. Individual cow weights and feed intakes, and calf growth and feed intake (including milk), were recorded. Milk production, milk composition and body composition were also measured so that correlated changes in efficiency of use of energy and nitrogen could be determined. Key results. The High-line cows were 18% (P < 0.05) heavier than the Low-line cows at the start and consumed 7% (P < 0.05) more feed than did the Low-line cows. Feed efficiency of the cow–calf unit was 12% higher (P < 0.05) in the High-line cows and calves than in the Low-line cows and calves. When compared on the basis of feed used relative to their weight and weight gain there was no difference (P > 0.05) between the selection lines. Divergent selection was accompanied by a change in body composition, with the High-line cows containing proportionally less protein and more fat in their bodies than did the Low-line cows. There was no evidence for change in the efficiency of feed energy use, but there was a 10% (P < 0.05) improvement in nitrogen efficiency of the cow–calf unit in the High-line compared with the Low-line. Conclusions. Divergent selection for weight gain led to a correlated change in cow size and cow feed requirements. Implications. This experiment supported the consensus among earlier reviews that there is little evidence that selection for growth rate or size, without moderating change in mature-cow weight, is associated with improved efficiency of feed energy use in maternal beef breeds.