Heat stress (HS) negatively affects dry matter intake (DMI), milk yield (MY), feed efficiency (FE), and free water intake (FWI) in dairy cows, with detrimental consequences to animal welfare, health, and profitability of dairy farms. Absolute enteric methane (CH₄) emission, yield (CH₄/DMI), and intensity (CH₄/MY) may also be affected. Therefore, the goal of this study was to model the changes in dairy cow productivity, water intake, and absolute CH₄ emissions, yield, and intensity with the progression (days of exposure) of a cyclical HS period in lactating dairy cows. Heat stress was induced by increasing the average temperature by 15°C (from 19°C in the thermoneutral period to 34°C) while keeping relative humidity constant at 20% (temperature-humidity index peaks of approximately 83) in climate-controlled chambers for up to 20 d. A database composed of individual records (n = 1,675) of DMI and MY from 82 heat-stressed lactating dairy cows housed in environmental chambers from 6 studies was used. Free water intake was also estimated based on DMI, dry matter, crude protein, sodium, and potassium content of the diets, and ambient temperature. Absolute CH₄ emissions was estimated based on DMI, fatty acids, and dietary digestible neutral detergent fiber content of the diets. Generalized additive mixed-effects models were used to describe the relationships of DMI, MY, FE, and absolute CH₄ emissions, yield, and intensity with HS. Dry matter intake and absolute CH₄ emissions and yield reduced with the progression of HS up to 9 d, when it started to increase again up to 20 d. Milk yield and FE reduced with the progression of HS up to 20 d. Free water intake (kg/d) decreased during the exposure to HS mainly because of a reduction in DMI; however, when expressed in kg/kg of DMI it increased modestly. Methane intensity also reduced initially up to d 5 during HS exposure but then started to increase again following the DMI and MY pattern up to d 20. However, the reductions in CH₄ emissions (absolute, yield, and intensity) occurred at the expense of decreases in DMI, MY, and FE, which are not desirable. This study provides quantitative predictions of the changes in animal performance (DMI, MY, FE, FWI) and CH₄ emissions (absolute, yield, and intensity) with the progression of HS in lactating dairy cows. The models developed in this study could be used as a tool to help dairy nutritionists to decide when and how to adopt strategies to mitigate the negative effects of HS on animal health and performance and related environmental costs. Thus, more precise and accurate on-farm management decisions could be taken with the use of these models. However, application of the developed models outside of the ranges of temperature-humidity index and period of HS exposure included in this study is not recommended. Also, validation of predictive capacity of the models to predict CH₄ emissions and FWI using data from in vivo studies where these variables are measured in heat-stressed lactating dairy cows is required before these models can be used.

Quantifying the impact of thermal stress on milk yields is essential to effectively manage present and future risks in dairy systems. Despite the existence of numerous heat indices designed to communicate stress thresholds, little information is available regarding the accuracy of different indices in estimating milk yield losses from both cold and heat stress at large spatio-temporal scales. To address this gap, we comparatively analyzed the performance of existing thermal indices in capturing US milk yield response to both cold and heat stress at the national scale. We selected four commonly used thermal indices: the Temperature and Humidity Index (THI), Black Globe Humidity Index (BGHI), Adjusted Temperature and Humidity Index (THIadj), and Comprehensive Climate Index (CCI). Using a statistical panel regression model with observational and reanalysis weather data from 1981-2020, we systematically compared the patterns of yield sensitivities and statistical performance of the four indices. We found that the US state-level milk yield variability was better explained by the THIadj and CCI, which combine the effects of temperature, humidity, wind, and solar radiation. Our analysis also reveals a continuous and nonlinear responses of milk yields to a range of cold to heat stress across all four indices. This implies that solely relying on fixed thresholds of these indices to model milk yield changes may be insufficient to capture cumulative thermal stress. Cold extremes reduced milk yields comparably to those impacted by heat extremes on the national scale. Additionally, we found large spatial variability in milk yield sensitivities, implying further limitations to the use of fixed thresholds across locations. Moreover, we found decreased yield sensitivity to thermal stress in the most recent two decades, suggesting adaptive changes in management to reduce weather-related risks.

The objective of this meta-analysis was to quantitatively summarize variations in urea kinetics related to ruminant species, diet composition, and ruminal fermentation. A database of 31 studies measuring urea recycling kinetics were used to derive 2 sets of linear mixed-effects regression models. Study was used as a random intercept and regressions were weighted by 1 divided by the standard error of the mean observation. Models were compared, when appropriated, using the concordance correlation coefficient, root estimated variance associated with study (σˆ_s) and error (σˆ_e) and corrected Akaike information criterion values. From a dietary standpoint, most response variables were affected by measures reflecting dietary crude protein [(CP; e.g., N-NH₃ or rumen-degradable protein (RDP)] and by variables reflecting dietary energy content [e.g., total digestible nutrients (TDN), dietary starch, or ruminal pH]. Dietary CP, N-NH₃, and TDN typically had positive slopes on urea N entry rate (UER; g/d and g/kg^0.75), whereas starch and TDN/RDP had negative slopes on UER (g/kg^0.75). On the other hand, increasing TDN increased gastrointestinal entry rate (GER; g/kg^0.75), whereas an opposite effect was observed for RDP. Increasing diet RDP content reduced the urea N returned to ornithine cycle (ROC; g/kg^0.75) in most models. Ruminal variables also reflected the importance of N and energy supplies. Ruminal ammonia concentration significantly affected ROC (g/d and g/kg^0.75), used for anabolism (UUA; g/kg^0.75), ROC:GER, UUA:GER, and the incorporation of recycled urea N into microbial N relative to gastrointestinal entry rate of urea. Ruminal pH significantly affected GER:UER and ROC:GER ratios. Total digestible nutrients had a positive slope on UUA (g/kg^0.75). Increasing the ratio of energy to protein (TDN:RDP) increased the GER:UER ratio, decreased the ROC:GER ratio, and increased the UUA:GER ratio and the incorporation of recycled urea N into microbial N relative to gastrointestinal entry rate of urea N. Comparison among models revealed that species was an important explanatory variable affecting most response variables. However, whether these differences are related to the intrinsic N metabolism of each species or due to the diet variation remains unclear. Understanding these differences could lead to improvements in N use efficiency in ruminant diets by formulating more precise low-N diets considering the particularities for each species.

Supplemental dietary rumen available fats show promise as enteric methane (eCH₄) mitigators for lactating dairy cows. However, concerns include variability in eCH₄ response and possible negative effects on dairy cow performance. Successful implementation of this mitigation option requires better prediction of responses specifically to rumen available fatty acids (FA) as well as understanding the modulating effects of other dietary and animal characteristics. Using meta-analytic and meta-regression techniques, 35 published studies with diet definition were used to assess changes in eCH₄ emissions and lactation performance associated with supplemental fat, specific supplemental rumen available FA types, and other dietary characteristics. Enteric CH₄ (g/d) was reduced by 3.77% per percentage unit of supplemental rumen available ether extract (EE). Supplemental rumen available PUFA (C18:2 and C18:3) and UFA (C18:1, C18:2, C18:3) mitigated eCH₄ (g/d) emissions in dairy cows by 6.88 and 4.65% per percentage unit increase, respectively. The anti-methanogenic effects of PUFA, MUFA and MCFA increased with correspondingly greater basal dietary levels of each FA type. Higher rumen-degradable starch (RDS; >18% DM) in the basal diet promoted greater reductions in eCH₄ yield (eCH₄/DMI, g/kg) with supplemental rumen available PUFA and UFA. Both milk fat percentage and yield (kg/d) were reduced with rumen available fat supplementation with a reduction of 7.8% and 6.0%, respectively, relative to control diets. Our results highlight the importance of determining basal levels of the rumen available FA before providing supplemental rumen available FA as an option for enteric eCH₄ mitigation. Dairy nutritionists can use estimates generated from this analysis to predict changes in eCH₄ emissions and dairy cow performance associated with dietary supplementation of rumen available EE and specific rumen available FA types for the purpose of eCH₄ mitigation.

Phosphorus (P) deficiency impairs productivity of ruminants and is the most expensive mineral used in supplements for cattle and buffalo under Brazilian grazing conditions. There is a need for the rational use of P, particularly under extensive grazing conditions, as this mineral resource is expensive and the world’s supplies are exhaustible. The diagnosis of P deficiency must start with careful and detailed history taking, followed by clinical examination of the herd and the use of ancillary tests. The latter are complementary to the diagnosis; they should be neither the first nor the most important steps when conducting a professional diagnosis. The indication of corrective or prophylactic measures necessarily involves the correct diagnosis of P deficiency (in herds), whether it be clinical or subclinical. This review discusses the main aspects related to P for cattle and buffaloes under grazing conditions in Brazil.