Impact of bovine somatotropin administration beginning at day 70 of lactation on serum metabolites, milk constituents, and production in cows previously exposed to exogenous somatotropin.

Metabolic and production responses are reported for 72 cows treated with bovine somatotropin (BST) for 30 days starting at day 70 of lactation. Of these 72 cows, 48 had been exposed in the preceding lactation to long-term treatment with BST at 3 dosages and 24 (controls) had not been given BST. Approximately half of the cows in each group were parity-2 cows, the rest were older. Comparisons between groups were made separately for parity-2, and older cows. Analyses, using pretreatment values of each variable as a covariate, indicated that older cows, but not parity-2 cows, significantly (P less than 0.05) increased milk production during treatment. Parity-2 cows, however, had a significantly higher milk fat percentage than controls following treatment. Cows treated with 51.6 or 86 mg BST/d in both parity groups had significantly higher serum-free fatty acids than controls. Estimated net energy balances were significantly lower for older treated cows, but did not significantly differ from controls for parity-2 treated cows. Older cows in the 86 mg of BST/d group tended to have higher concentrations of blood glucose than did older control-group cows. Treatment with BST did not significantly increase serum ketone concentrations in any group of animals, and none of the cows developed clinical ketosis during this period. Estimated net energy balance (ENEB) during treatment was a significant (P less than 0.05) covariate for free fatty acid concentrations in older cows and for milk fat percentage in parity-2 cows. Covariate adjusted analyses, using ENEB during treatment as a covariate, indicated that lipolytic stimuli already acting may be enhanced by treatment with BST, but a negative energy balance was not a necessary precondition for free fatty acid concentrations to increase following somatotropin treatment. Similarly, milk fat percentages for parity-2 treated cows were significantly (P less than 0.05) higher during treatment than controls when ENEB during treatment was used as a covariate. Increased milk fat concentrations in parity-2 treated cows were not associated with significant increases in the ratio of C18:C4-10 milk fatty acids, indicating that increased milk fat resulted from either an increase in incorporation of C18 fatty acids into milk fat coupled with an increase in de novo mammary synthesis of C4-10 milk fatty acids or an increase in C12-16 fatty acids that may arise either from increased tissue mobilization, from diet, or from de novo mammary synthesis.     

Searching for DNA markers for milk production and composition on chromosome 6 in sheep

Several milk protein polymorphisms are potential tools for selection in dairy ruminants. However, research results for dairy sheep are not as conclusive as those for goats or cattle and are often controversial. The main objective of this study was to find and later use molecular genetic markers in selection to improve milk production and milk composition in Awassi ewes. Chromosome 6 was chosen because several studies have reported the presence of significant quantitative trait loci (QTL) affecting milk production traits on ovine and bovine chromosome 6. Altogether, genotypes for 13 microsatellite loci were determined for 258 ewes, which were purebred Awassi or Awassi-Merino crosses. Phenotypic data were lactation yield of milk, milk fat, protein and lactose (kg), average milk protein and fat percentage and average somatic cell count. Five out of the 13 microsatellites showed significant association with at least one of the examined traits.     

Physiological basis of high milk production in cattle]

Milk yield is determined by number, size, synthetic and secretory capacity of mammary alveolar cells and low amounts of residual milk after milking. Initial milk yield and persistency of lactation are important. Important factors to achieve high milk yield are: optimal mammary development and preparation to lactation; maintenance and possibly increase in number and of synthetic capacity of alveolar cells during established lactation; sufficient availability of substrates for milk synthesis through high feed intake, readiness to mobilize body reserves (especially depot fat) and increased mammary blood flow; optimal milk let-down; regular and possibly more than twice milking/day. Favorable for a high milk yield are high circulating concentrations of somatotropin, possibly associated with increased production and levels of the insulin-like growth factor I, in the presence of low concentrations of and reduced sensitivity or responsiveness of target organs to insulin, reduced circulating levels of thyroid hormones and possibly enhanced conversion of thyroxine to triiodothyronine in the mammary gland. Enhanced sensitivity of depot fat to lipolytic hormones is favorable for high milk production.     

不同能量和蛋白水平日粮对西农萨能奶山羊泌乳性能的影响

为筛选西农萨能奶山羊泌乳期全价料最适能量和蛋白水平,选取体重(50.56±0.76)kg、胎次(2～3)、产奶量(2.12±0.10)kg/d相近和分娩日期与体况一致,并处于泌乳高峰期的健康西农萨能奶山羊36只,随机分为6组,每组6只。采用3×2因子随机区组试验设计6种日粮,NE水平为12.64、13.08和13.52MJ/d,CP水平为16.13%和17.82%。试验期84d,共分4个阶段,每阶段21d。结果表明:日粮NE和CP水平对奶山羊DMI和血液生化指标没有显著影响,但存在交互作用,NE 13.52MJ/d和CP16.13%处理组的DMI最高。随日粮NE和CP水平升高泌乳量增加,其中CP提高可显著增加泌乳量(P=0.041);而乳脂肪、乳蛋白、乳糖、干物质和非脂乳固体等乳成分指标则降低,其中CP提高可显著降低乳蛋白(P=0.013)和非脂乳固体(P=0.031);日粮NE和CP水平对奶山羊泌乳性能的影响存在交互作用,NE 13.52MJ/d和CP17.82%时泌乳量最高,而NE 13.08MJ/d和CP16.13%时乳品质最好。综合各项指标并考虑生产实际,得出西农萨能奶山羊泌乳期日粮适宜的NE和CP水平分别为13.08MJ/d和CP16.13%。     

Effect of feed restriction on dairy cow milk production: a review

In the dairy cow, negative energy balance affects milk yield and composition as well as animal health. Studying the effects of negative energy balance on dairy cow milk production is thus essential. Feed restriction (FR) experiments attempting to reproduce negative energy balance by reducing the quantity or quality of the diet were conducted in order to better describe the animal physiology changes. The study of FR is also of interest since with climate change issues, cows may be increasingly faced with periods of drought leading to a shortage of forages. The aim of this article is to review the effects of FR during lactation in dairy cows to obtain a better understanding of metabolism changes and how it affects mammary gland activity and milk production and composition. A total of 41 papers studying FR in lactating cows were used to investigate physiological changes induced by these protocols. FR protocols affect the entire animal metabolism as indicated by changes in blood metabolites such as a decrease in glucose concentration and an increase in non-esterified fatty acid or β-hydroxybutyrate concentrations; hormonal regulations such as a decrease in insulin and insulin-like growth factor I or an increase in growth hormone concentrations. These variations indicated a mobilization of body reserve in most studies. FR also affects mammary gland activity through changes in gene expression and could affect mammary cell turnover through cell apoptosis, cell proliferation, and exfoliation of mammary epithelial cells into milk. Because of modifications of the mammary gland and general metabolism, FR decreases milk production and can affect milk composition with decreased lactose and protein concentrations and increased fat concentration. These effects, however, can vary widely depending on the type of restriction, its duration and intensity, or the stage of lactation in which it takes place. Finally, to avoid yield loss and metabolic disorders, it is important to identify reliable biomarkers to monitor energy balance.     

中国荷斯坦牛RPL23A和ACACB基因对产奶性状的遗传效应分析

作者所在团队前期通过奶牛乳腺上皮组织转录组测序及荷斯坦公牛全基因组重测序研究发现RPL23A和ACACB基因是奶牛乳蛋白和乳脂性状的候选功能基因,本研究旨在探究这两个基因是否对奶牛产奶性状具有显著遗传效应。以北京地区7个牧场的1059头中国荷斯坦母牛为试验群体,采集尾根静脉血并提取基因组DNA,通过飞行时间质谱方法检测SNP位点基因型,利用SAS9.4软件的MIXED过程进行关联分析。结果表明,RPL23A基因的SNP位点g.20146771C>T与第1泌乳期5个产奶性状达到显著或极显著关联(P=0.0001~0.0416),其优势等位基因为T;ACACB基因的g.63878254T>C位点与第1泌乳期产奶量、乳脂量和乳蛋白量呈极显著关联(P<0.01),其优势等位基因为C;g.63962768G>A位点与第1泌乳期产奶量、乳脂量、乳脂率和乳蛋白率关联显著或极显著(P=0.0001~0.0391),其优势等位基因为A。综上,RPL23A基因主要影响中国荷斯坦牛产奶量和乳蛋白,ACACB基因对产奶量和乳脂具有显著遗传效应,3个SNP位点可考虑作为遗传标记用于标记辅助选择培育奶牛高乳蛋白乳脂新品系和选育提高。     

Influence of race and crossbreeding on casein micelles size

Casein (CN) micelles are colloidal aggregates of protein dispersed in milk, the importance of which in the dairy industry is related to functionality and yield in dairy products. The objective of this work was to investigate the correlation of milk CN micelles diameter from Holstein and Zebu crossbreds with milk composition (protein, fat, lactose, total and nonfat solids and milk urea nitrogen), somatic cell count (SCC), age, lactation stage and production. Average casein micelles diameters of milk samples obtained from 200 cows were measured using photon correlation spectroscopy and multiple regression analysis was used to find relationship between variables. CN micelle diameter, SCC and nonfat solids were different between animals with different Holstein crossbreed ratios, which suggests influence of genetic factors, mammary gland health and milk composition. Overall, results indicate the potential use of CN micelle diameter as a tool to select animals to produce milk more suitable to cheese production.     

饲粮全棉籽比例对奶牛泌乳性能及乳脂脂肪酸组成的影响

本试验旨在研究饲粮全棉籽比例对奶牛泌乳性能和乳脂脂肪酸组成的影响.试验采用4×4拉丁方试验设计,选择8头泌乳早期的经产中国荷斯坦奶牛(产奶量、胎次、产犊日期接近,健康无疾病),分为4组,每组2个重复,每个重复1头牛.A组饲喂基础饲粮,B、C和D组分别饲喂含5％、10％、15％全棉籽的试验饲粮.每期试验28 d,其中预试期为7d,正试期为21 d.结果表明:随着饲粮全棉籽比例的增加,奶牛产奶量有提高趋势(P＞0.05),校正乳产量极显著提高(P＜0.01).全棉籽极显著地提高了奶牛的乳脂率(P＜0.01),对乳蛋白率、乳糖率和乳非脂固形物率无显著影响(P＞0.05).全棉籽极显著降低了饱和脂肪酸( SFA)的含量(P＜0.01),不同程度地提高了乳中多不饱和脂肪酸( PUFA)和单不饱和脂肪酸(MUFA)的含量(P＜0.05或P＜0.01);短链脂肪酸和中链脂肪酸含量都显著降低(P＜0.05),长链脂肪酸含量有不同程度的提高(P＜0.05或P＜0.01).结果提示,饲粮中添加全棉籽可以提高奶牛产奶量,优化乳脂脂肪酸组成,改善乳脂品质,并以10％为泌乳早期奶牛最佳饲粮全棉籽比例.     

复合菌培养物对奶牛产奶性能及体细胞数的影响

试验旨在研究日粮中添加复合菌培养物对奶牛产奶量、乳品质和体细胞数的影响。选择30头荷斯坦奶牛,分为对照组和试验组,对照组饲喂基础日粮,试验组在基础日粮中添加复合菌培养物400 g/(头·d),试验期为37 d。测定指标包括产奶量、乳脂率、乳蛋白率及体细胞数。结果表明,试验组奶牛产奶量比对照组增加6.84%(P>0.05);乳汁体细胞数下降率比对照组提高29.78%(P<0.05);两组奶牛乳脂、乳蛋白之间无明显差异(P>0.05)。本试验结果表明,复合菌培养物在降低体细胞数和改善奶牛产奶性能方面具有良好的效果。     

线性模型对影响奶牛产奶性能的主要相关因素分析

利用一般线性模型研究各种因素对奶牛产奶性能的影响。牛场、胎次和产犊季节对奶牛产奶量影响差异极显著(P0.01),随着奶牛胎次的增加,奶牛产奶量增加;夏季产犊的奶牛产奶量最低,冬季产犊的最高;体细胞计数对奶牛产奶量没有显著性影响(P0.05),但随着体细胞数的增加,产奶量下降。牛场、胎次和体细胞计数对乳脂率有极显著的影响(P0.01),第三牛场平均乳脂率为4.38%,显著高于其他三个牛场;随着胎次的增加,乳脂率有下降趋势;随体细胞数增加,乳脂率升高;产犊季节对奶牛乳脂率没有显著影响(P0.05)。牛场、产犊季节和体细胞数对乳蛋白率的影响极显著(P0.01),体细胞数增加,乳蛋白率升高;夏季和秋季产犊的奶牛乳蛋白率较高,春季和冬季较低;胎次对乳蛋白率没有显著影响(P0.05)。表型相关分析表明:SCC与产奶量呈显著负相关(r=-0.158,P0.05),SCS与产奶量相关性接近显著水平(r=-0.140,P=0.055)。SCC/SCS与乳脂率、乳蛋白率呈正相关,但未达到显著性水平(P0.05)。     

中国荷斯坦奶牛RPL23A、ACACB基因3个SNPs检测及其与产奶性状的关联分析

旨在开展奶牛群体高乳成分功能基因的验证与筛选,利用微流控芯片自主选育技术分析北京地区母牛群体高乳蛋白、高乳脂基因频率,同时分析基因多态性及其与产奶性状的相关性。本研究对北京地区8个大型奶牛场1 596头中国荷斯坦奶牛RPL23A、ACACB基因的多态性进行了检测,所有个体均为3胎以内的泌乳牛,收集每头牛所有测定日的产奶量、乳脂率、乳蛋白率等数据,同时对3个多态位点不同基因型与产奶性状进行了关联分析。RPL23A基因的SNP位点g.20146771G>A,在第一泌乳期,与产奶量、乳脂量、乳脂率和乳蛋白量均达到极显著关联（P<0.01）,第二泌乳期,g.20146771G>A与5个产奶性状均呈极显著关联（P<0.01）。ACACB基因的SNP位点g.63962768C>T,在第一泌乳期,与产奶量、乳脂量、乳脂率和乳蛋白量均呈极显著关联（P<0.01）,在第二泌乳期,与产奶量、乳脂量、乳脂率和乳蛋白率呈极显著关联（P<0.01）。结果表明,RPL23A、ACACB基因可以作为影响中国荷斯坦奶牛产奶性状的候选基因用于标记辅助选择,以上基因位点可能通过直接或间接的途径影响奶牛的乳脂或乳蛋白性状,对产奶性状起到重要调控作用。本研究为荷斯坦奶牛后续的标记辅助选择奠定了良好基础。     

西农萨能奶山羊泌乳期产奶量、乳成分及血液生理生化指标变化规律研究

旨在探索西农萨能奶山羊泌乳期生产性能、乳成分、血液生理生化指标和营养物质摄入量等变化规律及其相互关系.本研究选取15只体重、胎次、产奶量、分娩日期相近,体况健康的西农萨能奶山羊.单圈饲喂54周,试验期内准确测定个体生产性能、乳成分、血液生理生化指标和营养物质摄入量等,建立各项指标随泌乳期变化规律模型及泌乳期内指标间的相...     

New developments on the galactopoietic role of prolactin in dairy ruminants

In most mammals, prolactin (PRL) is essential for maintaining lactation and its suppression strongly inhibits lactation. However, the involvement of PRL in the control of ruminant lactation is less clear because inconsistent effects on milk yield have been observed with short-term suppression of PRL by bromocriptine. By contrast, in vitro studies have provided evidence that PRL helps to maintain the differentiation state and act as a survival factor for mammary epithelial cells. Therefore, a series of experiments were conducted to assess the galactopoietic role of PRL. In a first experiment, daily injections of the PRL inhibitor quinagolide reduced milking-induced PRL release and induced a faster decline in milk production. Milk production was correlated with PRL released at milking. Quinagolide reduced mammary cell activity, survival, and proliferation. During the last week of treatments, differential milking (1× vs 2×) was applied. The inhibition of milk production by quinagolide was maintained in the udder half that was milked 2× but not in the udder half milked 1×, suggesting that the response to PRL is modulated at the gland level. In a second experiment, cows were injected with quinagolide, quinagolide + injection of bovine PRL at milking time, or water. As in the first experiment, quinagolide reduced milk, protein, and lactose yields. Although PRL injections at milking time were not sufficient to restore milk yield, they tended to increase milk protein and lactose yields and increased the viability of milk-purified mammary epithelial cells. Recently, we investigated the use of quinagolide at drying off. Treating late-lactation cows with quinagolide decreased milk production within the first day of treatment and induced faster increases in somatic cells and bovine serum albumin content in mammary secretions after drying off, which indicates an acceleration of mammary gland involution. In conclusion, these data, combined with data from other studies, provide a good body of evidence indicating that PRL is galactopoietic in dairy cows. However, the response to PRL appears to be modulated at the mammary gland level.     

The association between first DHI milk-test parameters and subsequent displaced abomasum diagnosis in dairy cows

First Dairy Herd Improvement Corporation (DHI) test milk that was sampled prior to displaced abomasum (DA) diagnosis was used to examine the association between milk yield (kg), fat (%), protein (%), or fat/protein ratio and DA in dairy cows. 27 DA cases were matched to 3 controls per case by herd and calving date. Logistic regression was used to examine the effect of each milk parameter on the odds of DA, adjusting for lactation number and days in milk (DIM) at test. Higher odds of DA diagnosis was found with lower milk yield, higher milk fat percentage, lower milk protein percentage and higher fat/protein ratio. We conclude that milk yield, fat percentage, protein percentage and fat/protein ratio may be used to detect cows at risk for DA. Our findings also indicate that cows diagnosed with DA were energy deficient prior to DA diagnosis.     

用大豆皮替代日粮中部分玉米对奶牛产奶量和乳成分的影响

选用36头产奶牛,按产奶量、泌乳月和胎次配对的原则随机均分为对照组和试验组。试验处理为大豆皮替代饲粮30％的玉米。正试期90天。结果表明,奶牛日产奶量、4％乳脂率校正奶(FCM)产量和乳脂率均较对照组有明显提高,处理组日产奶量提高1.5千克;由于乳脂率的差异,4％FCM产量提高2.7千克(P<0.08)。乳脂率提高0.32个百分点(P<0.05)。结论:在奶牛饲粮中用大豆皮替代30％玉米时,可以提高产奶量和乳脂率;大豆皮可作为奶牛较好的精料替代成分。     

Negative effects of long-term feeding of high-grain diets to lactating goats on milk fat production and composition by regulating gene expression and DNA methylation in the mammary gland

Background: It is well known that feeding a high concentrate(HC) diet to lactating ruminants likely induces subacute ruminal acidosis(SARA) and leads to a decrease in milk fat production. However, the effects of feeding a HC diet for long periods on milk fatty acids composition and the mechanism behind the decline of milk fat still remains poorly understood. The aim of this study was to investigate the impact of feeding a HC diet to lactating dairy goats on milk fat yield and fatty acids composition with an emphasis on the mechanisms underlying the milk fat depression. Seventeen mid-lactating dairy goats were randomly allocated to three groups. The control treatment was fed a low-concentrate diet(35% concentrate, n = 5, LC) and there were two high-concentrate treatments(65% concentrate, HC), one fed a high concentrate diet for a long period(19 wks, n = 7, HL); one fed a high concentrate diet for a short period of time(4 wk, n = 5, HS). Milk fat production and fatty acids profiles were measured. In order to investigate the mechanisms underlying the changes in milk fat production and composition,the gene expression involved in lipid metabolism and DNA methylation in the mammary gland were also analyzed.Results: Milk production was increased by feeding the HC diet in the HS and HL groups compared with the LC diet(P 0.01), while the percentage of milk fat was lower in the HL(P 0.05) but not in the HS group. The total amount of saturated fatty acids(SFA) in the milk was not changed by feeding the HC diet, whereas the levels of unsaturated fatty acids(UFA) and monounsaturated fatty acids(MUFA) were markedly decreased in the HL group compared with the LC group(P 0.05). Among these fatty acids, the concentrations of C15:0(P 0.01), C17:0(P 0.01), C17:1(P 0.01), C18:1 n-9 c(P 0.05), C18:3 n-3 r(P 0.01) and C20:0(P 0.01) were markedly lower in the HL group, and the concentrations of C20:0(P 0.05) and C18:3 n-3 r(P 0.01) were lower in the HS group compared with the LC group. However, the concentrations of C18:2 n-6 c(P 0.05) and C20:4 n-6(P 0.05) in the milk fat were higher in the HS group. Real-time PCR results showed that the m RNA expression of the genes involved in milk fat production in the mammary gland was generally decreased in the HL and HS groups compared with the LC group. Among these genes, ACSL1, ACSS1 2, ACACA, FAS, SCD, FADS2, and SREBP1 were downregulated in the mammary gland of the HL group(P 0.05), and the expressions of ACSS2, ACACA, and FADS2 m RNA were markedly decreased in the HS goats compared with the LC group(P 0.05). In contrast to the gene expression, the level of DNA methylation in the promoter regions of the ACACA and SCD genes was increased in the HL group compared with the LC group(P 0.05). The levels of ACSL1 protein expression and FAS enzyme activity were also decreased in the mammary gland of the HL compared with the LC group(P 0.05).Conclusions: Long-term feeding of a HC diet to lactating goats induced milk fat depression and FAs profile shift with lower MUFAs but higher SFAs. A general down-regulation of the gene expression involved in the milk fat production and a higher DNA methylation in the mammary gland may contribute to the decrease in milk fat production in goats fed a HC diet for long time periods.     

影响西农萨能奶山羊产奶性状的非遗传因素分析

本试验旨在探究母羊出生年度、季节、胎次、泌乳阶段4个固定效应对西农萨能奶山羊产奶性状的影响。以西北农林科技大学萨能羊原种场2006-2018年的645只泌乳母羊为研究对象,每月采集乳样1次,采样日早、晚各采集1次,将两次乳样等比例混合后,采用乳成分分析仪测定乳脂率、乳蛋白率、乳糖率、总脂固形物、非脂固形物、密度、冰点和酸度共8个指标,结合羊场产奶量记录,采用固定效应模型,通过SAS 9.4软件进行表型的描述性统计分析,再采用GLM模型进行固定效应对产奶性状的影响分析。结果表明,西农萨能奶山羊平均300 d产奶量为507.67 kg,乳脂率为3.58%,乳蛋白率为3.20%,乳糖率为4.19%;总固形物含量为12.21%,非脂固形物含量为8.46%;出生年度、胎次对产奶量影响极显著（P<0.01）,出生季节对产奶量影响不显著（P>0.05）;泌乳阶段、出生年度对乳脂率等8个乳成分性状均有极显著影响（P<0.01）,胎次对除乳蛋白率和总固形物的其他6个乳成分指标均存在极显著影响（P<0.01）。综合以上试验结果,胎次及泌乳阶段是影响西农萨能奶山羊产奶性状的两种主要非遗传因素,第3、4胎母羊产奶性能最佳,泌乳早期的乳品质更优,揭示了对种群进行良种选育工作和羊饲养管理的重要性,也为后期进行奶山羊经济性状遗传评估和群体遗传改良提供理论基础。     

Economic evaluation of the method of gradual formation of groups of dairy cows from primiparae free of infectious mastitis]

Employing the turnover of a dairy-cow herd with primiparas to control infectious mastitis is beneficial because young dairy cows, as a rule free of inflammations of the mammary gland, are not included among dairy cows already infected or suspected of being infected and so it is possible to form groups of dairy cows free of mastitis. For this intention an agricultural enterprise was chosen in potato production region I. In the given period 316 to 336 dairy cows of Bohemian Spotted breed were followed. To express the impact of the infection on the milk efficiency and to draw economic conclusions on the efficiency of treating infectious mastitis we observed the total milk yield of the different dairy cows in one lactation. In the given enterprise unsuccessful treatment of an infected dairy cow in three lactations resulted in a total loss of 4,648 Kcs. Therefore from the economic aspect it appears to be profitable to replace productive, but ill, older dairy cows by healthy first-calvers because the negative effect of the mastitis on the production of milk increases with every successive lactation.     

Anthelmintic treatment of dairy cows and its effect on milk production.

The results of more than 80 experiments on gastrointestinal parasitism and the impact of anthelmintic treatment on milk production in dairy cattle were reviewed. Abattoir surveys of culled dairy cows, faecal egg counts in milking cows, and serological tests and worm counts in cull cows in milk production studies were collated to assess the level of parasitism in dairy herds. The studies were divided into four general categories: induced infections in previously uninfected cattle; naturally infected cattle treated in mid-lactation; naturally infected cattle treated one to three times during the dry period and/or just before or just after parturition; and naturally infected cattle treated repeatedly from early lactation or given strategic treatments throughout the year. In most studies, the milk production of anthelmintic-treated cattle was compared with that of untreated controls. The anthelmintics investigated included members of the organophosphate, benzimidazole, imidazothiazole and macrocyclic lactone groups. The number of experiments in which the medicated (or uninfected) group had a higher milk yield was compared with the number of experiments in which the control (or infected) group had a higher yield. Overall, the studies demonstrated that grazing dairy cattle are likely to be infected with gastrointestinal nematode parasites, usually Ostertagia ostertagi and Cooperia species. These infections may be present as inhibited larvae, and a periparturient or spring rise is associated with their emergence. There is, at present, no reliable means of determining whether a cow or a herd may be parasitised subclinically at a level sufficient to interfere with milk production. In 70 of 87 experiments (80 per cent) there was an increase in milk production (P < 0.001) after anthelmintic treatment, with a median increase of 0.63 kg/day. In each of the four trial categories, a majority of the studies showed that anthelmintic treatment increased milk production. The yield of milk fat by the medicated cows was greater than by the controls in 26 of the 35 experiments in which that variable was studied (P < 0.01).